After shakedown in Long Island Sound, "Grouper" sailed for Pearl Harbor on 30 March 1942 to join the Pacific Submarine Force, which was to play havoc on Japanese shipping. Before departing for her first war patrol, "Grouper" was assigned to the submarine screen which ringed the area as the American and Japanese fleets clashed in the decisive Battle of Midway. Patrolling the fringe of the fighting on 4 June, "Grouper" sighted two burning enemy aircraft carriers, but could not close for attack because of heavy air cover. On that day, she was strafed by fighter planes and driven deep in a series of aircraft and destroyer attacks which saw over 170 depth charges and bombs dropped on the novice submarine.