While on intelligence duty he intercepted a shortwave radio message in plain English that 17 troop transports were en route to England, having cleared the port of Freetown, Sierra Leone.

[1] He rented a second-story apartment that overlooked Pearl Harbor and would often wander around the island of Oahu, taking notes on fleet movements and security measures.

[3] He rented small airplanes at John Rodgers Airport and flew around, observing U.S. installations; he also dove under the harbor using a hollow reed as a breathing device.

He and Seki agreed that, while Hawaii should be the "easiest place" to carry out such work in view of the large Japanese population, both looked upon the locals with disdain.

"[T]hose men of influence and character who might have assisted me in my secret mission were unanimously uncooperative...."[6] Instead he made "regular use of a Japanese-American military veteran, a driver in the employ of the consulate general for over 20 years, whom he valued for his being “trusted by the Americans.” Another Japanese-American working at the consulate general bought at least one airplane ticket in his own name for Yoshikawa, who wished to avoid suspicion by buying too many tickets himself.

"[7] Although he had no knowledge of a planned attack on Naval Station Pearl Harbor, Yoshikawa assumed that the intelligence would help prepare for such an eventuality and worked tirelessly to that end.

[8] When he heard the "East wind, rain" code phrase on the short wave radio bringing the news from Tokyo to signal that an attack against the United States was to proceed, Yoshikawa destroyed all evidence of his activities.

When the war ended and Japan was occupied by U.S. forces, he went into hiding (disguised as a Buddhist monk) for fear of being prosecuted for his role in the Pearl Harbor attack.

While the Yoshikawa case was used to retroactively justify the decision to intern Japanese Americans, he claimed that he distrusted the Japanese-American community and that it was loyal to America over Japan.