In combat, the Defiant was found to be reasonably effective at destroying bombers but was vulnerable to the "Luftwaffe"s more manoeuvrable, single-seat Messerschmitt Bf 109 fighters. The lack of forward-firing armament proved to be a great weakness in daylight combat and its potential was realised only when it was converted to a night fighter. It eventually equipped thirteen squadrons in this role, compared to just two squadrons as a day-fighter. In mid-1942 it was replaced by better performing night-fighters, the Bristol Beaufighter and de Havilland Mosquito.