Because of his expertise on the U.S. Navy, Yoshikawa was sent to Hawaii posing as a vice-consul named Tadashi Morimura (森村 正 "Morimura Tadashi"), arriving on March 27, 1941, with Nagao Kita (喜多 長雄 "Kita Nagao"), the new Japanese Consul-General aboard the liner "Nitta Maru". 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. 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 gathered information by taking the Navy's own harbor tugboat and listening to local gossip. He worked closely with German Abwehr agent Bernard Kuehn, as well as another former Etajima graduate, Kokichi Seki (関 興吉 "Seki Kō'kichi"), an untrained spy who served as the consulate's treasurer.