The flotilla operated mostly various marks of the Type VII U-boat and it concentrated its efforts mainly in the North Atlantic, against convoys to and from Great Britain. The flotilla served from Brest until the base was threatened with being captured by American forces. The last of the flotilla's U-boats, the U-256 left Brest on 4 September 1944 for Bergen, Norway, and this marked the end of the 9th Flotilla.