In 1941 there were 64(60) Japanese submarines, during the war another 126 were added.
Three types of boats were innitially operated the large I boats, mediums and the smaller coastal RO series.
Four vessels were designated as minelayers, three massive submarine were designed to act as carriers and the last twenty six boats built were transport vessels. Smaller vessels were also operated two man midget submarines of which there were several hundred.
These submarines sank 184 merchantmen, 2 carriers, 2 cruisers, 10 destroyer escorts and a few other naval vessels. The main kills were the US carrier Wasp, finishing off the US carrier Yorktown, the US escort carrier Liscombe bay sunk off Tarawa, the light cruiser Juneau in the Solomons and the US heavy cruiser Indianapolis.
Their own losses amounted to 129(112) boats, 70 to surface ships, 18 to aircraft, 19 to enemy submarines and 22 to other reasons.
In the first month of the war 11 out of 12 Japanese submarines on patrol sunk 8 vessels totalling 40,700 tons and damaged a further 7.