"Harder" reconnoitered Tawi-Tawi anchorage 11 June and sighted additional enemy cruisers and destroyers. At 16:00 she headed for the open sea and that night transmitted her observations which were of vital importance to Admiral Raymond A. Spruance's fleet prior to the decisive Battle of the Philippine Sea. "Harder" steamed to Darwin on 21 June for additional torpedoes, and, after patrolling the Flores Sea south of the Celebes Islands (with Admiral Ralph Christie aboard), she ended the patrol at Darwin on 3 July.