[2] She had her first success on 16 June 1943, when she torpedoed the 8,078-gross register ton British armed motor tanker MV San Ernesto — which was on a voyage in ballast from Sydney, Australia, to Abadan, Iran — southeast of the Chagos Archipelago.

[2] The derelict San Ernesto herself remained afloat, drifting 2,000 nautical miles (3,700 km; 2,300 mi) across the Indian Ocean before eventually running aground on the west side of Nias Island off Sumatra in the Japanese-occupied Netherlands East Indies at 01°15′N 097°15′E / 1.250°N 97.250°E / 1.250; 97.250 (MV San Ernesto).

[2] On 19 June 1943, I-37 hit the 7,176-ton American Liberty ship SS Henry Knox — bound from Fremantle, Australia, to Bandar Shapur with an 8,200-ton Lend-Lease cargo of fighter aircraft, tanks, and explosives destined for the Soviet Union — with one torpedo in her port side at 01°00′N 071°15′E / 1.000°N 71.250°E / 1.000; 71.250.

[2] Her navigator interrogated the survivors in the boat about their cargo, route, and destination, and about any Allied vessels they had encountered in the area, after which the Japanese ordered the men in the lifeboat to pass various items to I-37 via a handline.

[2] On 11 October 1943, I-37′s floatplane reconnoitered the harbor at Diego Suarez, Madagascar, its crew reporting the anchorage to be heavily guarded.

[2] Northwest of Madagascar, I-37 torpedoed and sank the Greek 3,404-gross register ton merchant ship SS Faneromeni on 23 October 1943.

[2] Just after sunset on 27 November 1943, at 12:40 Zulu Time, I-37 torpedoed the Norwegian 9,972-gross register ton armed tanker MV Scotia, which had separated from Convoy PB-64 to proceed independently during a voyage from Bahrain to Melbourne, Australia, with a cargo of diesel oil.

[2] On 15 December, she went into drydock at the naval base at Seletar in Singapore for an overhaul;[2] the same day, Submarine Division 14 was abolished and she was attached directly to the 8th Fleet.

[2][12] I-37′s medical officer interrogated the men in the lifeboats,[2] and I-37 brought Captain Hill aboard as a prisoner-of-war,[2][13] forcing him to surrender his briefcase, which contained about fifty diamonds and sapphires.

[16] At 14:00, 3+1⁄2 hours after torpedoing British Chivalry, I-37 finally ceased fire and headed off to the east after killing 13 men and wounding five on the boats and in the water.

[2][18] While I-37 was on the surface in the Arabian Sea 200 nautical miles (370 km; 230 mi) west of Diego Garcia at 20:30 on 26 February 1944, her lookouts sighted the British 5,189-gross register ton armed motor vessel MV Sutlej, which, after departing Aden on 15 February as part of a convoy,[19] had detached from the convoy on 20 February midway across the Arabian Sea to proceed independently during a voyage from Kosseir, Egypt, to Fremantle, Australia, with a cargo of 9,700 tons of rock phosphates and mail.

[21] She brought the raft alongside, and her medical officer, speaking from the bridge, interrogated the men aboard it about Sutlej′s identity, cargo, departure port, and destination and attempted to identify Sutlej′s master.

[23] I-37 fired first at the raft, inflicting no casualties on its occupants, and then spent an hour moving systematically through the area, her crew machine-gunning every man floating in a life jacket they could find[23] while calling for Sutlej′s master and chief engineer to give themselves up.

[2] At 11:30 on 29 February 1944, the submerged I-37 fired two torpedoes at the British 7,005-gross register ton armed cargo steamer SS Ascot — carrying a 9,000-ton general cargo of pig iron, paraffin wax, gunnies, linseed oil, coconuts, and fiber and, according to different sources, making a voyage either from Calcutta in British India to Port Louis, Mauritius,[2] or from Colombo, Ceylon, to Diego Suarez, Madagascar, and then to Fremantle[26] — in the Indian Ocean 800 nautical miles (1,500 km; 920 mi) northwest of Diego Suarez.

[2][27] One hit Ascot in the forward part of her engine room in her starboard side, killing four crewmen, knocking down both of her transmitter aerials, destroying two of her lifeboats, and bringing her to a stop at 05°S 063°E / 5°S 63°E / -5; 63.

[2][29] I-37 surfaced 2,000 yards (1,800 m) off Ascot′s starboard quarter,[2] circled her once, and fired at Ascot with her deck gun for about 15 minutes, scoring no hits.

[33] Ascot′s last seven survivors — four crewmen and three gunners — climbed back aboard the raft, from which the Dutch steamer or motor vessel (according to different sources) Straat Soenda rescued them on 3 March 1944.

[2][34] On 3 March 1944, I-37 launched her floatplane for an armed reconnaissance flight over the Chagos Archipelago, carrying two 60-kilogram (132 lb) bombs.

[2] Along the way, at 23:00 on 9 March she stopped an Indian junk making a voyage from Colombo, Ceylon, to Cape Town, South Africa, allowing it to proceed after discovering about 100 women and children were aboard.

[2] After sunset on 15 March, her floatplane made the flight, its crew reporting an aircraft carrier, two heavy cruisers, and three destroyers in the harbor.

[2] Although bad weather persisted on 7 April, her plane made its flight that night, its crew reporting more than 60 merchant ships in the harbor at Mombasa.

[2] Since being taken prisoner on 22 February, British Chivalry′s Captain Hill had refused to answer his captors' questions aboard I-37 and I-37′s crew had occasionally taken him out on deck blindfolded and with his hands tied behind his back, threatening to shove him overboard.

[15] After I-37 arrived at Penang, he was placed on a starvation diet as punishment for taking up arms against the emperor of Japan, but after the Allies freed him at the end of the war, he refused to provide evidence against the Japanese.

[2] Around 08:00, when she was about 20 nautical miles (37 km; 23 mi) south of Penang, an explosion occurred about 110 yards (100 m) off her port bow, apparently the premature detonation of a naval mine laid either by a B-24 Liberator heavy bomber of the 7th Bombardment Group of the United States Army Air Force′s Tenth Air Force or by the Royal Navy submarine HMS Taurus.

[2] In January 1947, he pleaded guilty before an American military court in Yokohama for the war crimes he committed while in command of I-37 and was sentenced to seven years in prison with hard labor.

[35] After completion of her repairs, I-37 served as an antisubmarine warfare target in the anchorage at Lingga Island off Sumatra for ships of the 2nd Fleet between 09:00 and 13:30 Japan Standard Time on 21 July 1944.

[2] On 9 September 1944, she arrived at Kure, Japan, for a refit and modifications involving the removal of her hangar, aircraft catapult, and deck gun and the installation of fittings for her to carry four kaiten manned suicide torpedoes.

[2] On 7 November 1944, the commander of the 6th Fleet, Vice Admiral Shigeyoshi Miwa, advised crews at the kaiten base at Otsu Island in Tokuyama Bay on the coast of Japan of the plan for Operation Kikusui ("Floating Chrysanthemum"), in which I-37 and the submarines I-36 and I-47 were to launch kaiten attacks on the Allied naval anchorages at Ulithi Atoll and at Kossol Roads at Palau.

[2] Assigned to the Palau attack, I-37 embarked four kaitens and their pilots for the operation, and all three submarines departed the Otsu Island base on 8 November 1944.

[2] By sunset, a whaleboat from McCoy Reynolds had retrieved wood stenciled with Japanese characters, polished pieces of instrument cases, deck planking, and a piece of human flesh with bits of steel embedded in it from the water, and by the time darkness fell an oil slick extended over several square miles as additional debris came to the surface, marking the demise of I-37.