On 15 June 1944, the Marianas campaign began with the U.S. invasion of Saipan. Anticipating the invasion, on 13 June 1944 the commander of the 6th Fleet, Vice Admiral Takeo Takagi, ordered all available Japanese submarines to deploy east of the Mariana Islands. With her overhaul recently completed, "I-6" set out from Yokosuka bound for the waters off Saipan on 16 June 1944. At 22:33 that evening the Japanese 5,123-ton armed cargo ship "Toyokawa Maru" — a member of Convoy 3606, consisting of four merchant ships escorted by three coastal defense ships and a submarine chaser, which had left Ogasawara in the Bonin Islands on 14 June 1944 bound for Yokosuka — sighted "I-6" surfacing near the convoy. Mistaking her for an enemy submarine, "Toyokawa Maru" sounded a submarine alert, turned sharply toward "I-6", and rammed her on her starboard side just abaft her conning tower. "I-6" listed heavily, capsized, and sank a few minutes later, and "Toyokawa Maru" followed up by dropping depth charges and machine gunning the water where "I-6" sank. None of "I-6"′s crew of 104 survived.