On 10 May 1940, with the attack on France, the Squadron became part of the Advanced Air Striking Force and moved to France where it saw extensive action, stationed at airfields as Bétheniville, Anglure, Le Mans and Dinard. Sgt J.H. "Ginger" Lacey of 501 Squadron shot down three enemy aircraft in a single day to win the Croix de Guerre. (He later returned to England with five victories.) After the retreat from France through Saint Helier, Jersey, its battle-hardened pilots were reorganised at RAF Croydon and then moved on to RAF Middle Wallop and later RAF Gravesend (now Gravesend Airport). It subsequently served at RAF Kenley, south London, commanded by S/L Harry Hogan, until 17 December 1940 by which time the squadron had claimed 149 enemy aircraft destroyed. Success came at a high cost; in addition to the heavy losses suffered in France, the squadron lost 19 pilots killed during the Battle of Britain, more than any other squadron.