Chuyō, the last of the three to be converted, only ferried aircraft between Japan and the naval base at Truk before she was sunk by an American submarine in December 1943.

After finishing their repairs in 1944, the sisters combined convoy escort duties with their transport missions and often ventured as far south as Singapore.

[1] Kasuga Maru had been ordered by OSK and was fitting out when she was acquired by the IJN in 1940 and towed to Sasebo Naval Arsenal on 1 May 1941 to finish her conversion into an escort carrier.

[3] The ships carried 2,290 tonnes (2,250 long tons) of fuel oil that gave them a range of 6,500 nautical miles (12,000 km; 7,500 mi) at a speed of 18 knots (33 km/h; 21 mph).

[4][14] Taiyō, as the first ship completed, had an armament of six single 45-calibre 12 cm (4.7 in) 10th Year Type anti-aircraft (AA) guns in sponsons along the sides of the hull.

[16] Chūyō and Un'yō were equipped with eight more modern 40-caliber 12.7 cm (5 in) Type 89 dual-purpose guns in four twin mounts on sponsons along the sides of the hull.

[19] Completed before the start of the Pacific War, Taiyō ferried aircraft, supplies, and personnel between Japanese bases and trained naval aviators in between transport missions.

In September the ship transferred supplies and equipment from Truk to the Philippines, Formosa and Palau; on the return voyage she was torpedoed by an American submarine.

[6] Her conversion and work up completed by the end of June 1942, Un'yō made three voyages from Japan to Truk, Saipan, and Rabaul between July and October.

From late October to early January 1943, she ferried aircraft from Japan to the Philippines, Palau, the Dutch East Indies and Truk.

[24] Chu'yō's conversion was finished in November 1942 and she spent the next year exclusively ferrying aircraft to Truk, usually in company with one or the other of her sisters.

[25] After Taiyō's repairs were completed in November 1943, she was transferred to the Grand Escort Command and began a refit that lasted until April 1944.

While en route to Japan, part of her bow broke off in a storm and caused the forward end of the flight deck to collapse.

She was torpedoed by the submarine USS Rasher on 10 August which caused her aft avgas tank to explode; the carrier sank about half-an-hour later with heavy casualties.