Biddle assumed command of the 1st Brigade, 3rd Division before the Battle of Gettysburg began on July 1, 1863. The assignment of Maj. Gen. John F. Reynolds as commander of the army's left wing led to acting promotion of brigade commander Brig. Gen. Thomas A. Rowley to command of the division while Maj. Gen. Abner Doubleday led the corps. Colonel Biddle led the four regiments of the brigade in the first day's fighting on McPherson's Ridge and the subsequent withdrawal to Cemetery Ridge. His report on the first day's fighting describes the brigade's fighting as taking place under heavy artillery fire. When the brigade was flanked by Confederate infantry, Biddle led an unsuccessful counterattack. Later he received a head wound from a spent Minié ball when Col. Abner Perrin's brigade attacked the brigade's fall-back position on Seminary Ridge. Biddle had his head bandaged, and then returned to his troops.