The submarine's fifth patrol—from 13 June through 1 August—was off the coast of Indochina in the South China Sea. On 4 July, she intercepted and tracked a large freighter, but discovered the intended target was a hospital ship and held her fire. On 9 July, she intercepted and torpedoed a Japanese freighter. One of a pair of torpedoes struck home and the ship took a 15° list. As "Sailfish" went deep, a series of explosions were heard, and no further screw noises were detected. When the submarine surfaced in the area 90 minutes later, no ship was in sight. Though she was credited during the war with a ship, postwar examination of Japanese records confirmed no sinking in the area on that date.