On her first war patrol from 8 February to 4 April 1942, "Grampus" sank an 8636-ton tanker Kaijo Maru No.2 4 March 1942, the only kill of her short career, and reconnoitered Kwajalein and Wotje atolls, later the scene of bloody but successful landings. "Grampus"' second patrol en route to Fremantle, Australia, and her third patrol from that base were marred by a heavy number of antisubmarine patrol craft off Truk Lagoon and poor visibility as heavy rains haunted her path along the Luzon and Mindoro coasts.