The first production version received a more powerful engine than the prototype and was designated the 'A6M2'.  As it was first produced in 1940 - the Japanese year 5,700 - it became popularly known as the "Zero-Sen" ("Type 00 Fighter").  Two squadrons with 15 planes were sent to China in July 1940 for trials under operational conditions, and quickly eliminated all opposition.  The effectiveness of the Zero was urgently and emphatically reported to Washington by General Chennault, commanding officer of the Flying Tigers, but his report appears to have gone unnoticed. 

More than 400 A6M2 and A6M3 (clipped-wing) Zeros had been delivered by the time of the attack on Pearl Harbor. 
They had unprecedented range for a small aircraft, and their unrivalled maneuverability and powerful armament gave them decisive superiority over every opponent. This was startlingly demonstrated in the heavy Japanese carrier raids on Ceylon 5-9 April 1942, in which they were in action against British Hawker Hurricanes.  These were fighters which were regarded as outstandingly maneuverable(and which had proved their superiority in this respect to the Messerschmidt Bf-109s of the German Luftwaffe). Nonetheless the Hurricanes were easily out-turned by the raiding Zeros. 

At Midway many pilots were lost aboard the Japanese carriers,all four of which were set ablaze by the carrier-borne American Dauntless dive-bombers. In the protracted and bitter Guadalcanal campaign losses of aircrew mounted and the quality of Japanese pilots correspondingly declined.  Allied aircraft therefore achieved increasing success against the Zero. When, on top of this, much more modern and capable US aircraft - notably the Grumman F6F Hellcat and Vought F4U Corsair - appeared in the combat areas of the Pacific,  the A6M found itself outclassed. 

Mitsubishi therefore tried desperately to come up with a more effective version of the Zero. These efforts resulted in the A6M5 - the variant produced in the largest numbers.  However, the improvement it represented was not sufficient, and the Zero was never, after 1943, able to fight on equal terms with the best Allied aircraft.  However,  the A6M6c equipped with the combat-boosted Sakae 31 engine,  and the A6M8c equipped with 1,560 hp Kinsei 62 engine, were in 1945 able to give considerable trouble to the F4F and FM Wildcat fighters operating from US escort carriers.

Total production of the A6M came to 10,449 units.