(Imperial War Museum photo, scanned from Polmar and Carpenter's "Submarines of the Imperial Japanese Navy")

The C1 design was developed from the Type KD6.  These carried no aircraft, but they did have fittings for carrying a midget submarine aft.  This was later modified to accommodate a 46-foot daihatsu barge.  Their eight torpedo tubes were all forward and were served via two separate torpedo rooms, one above the other.

These five boats carried the five midget submarines that were deployed to assist the attack on Pearl Harbor.  All of the midgets were lost, and only one actually entered the harbor.  It fired its two torpedoes at a seaplane tender and a destroyer, but hit neither, and was promptly sunk by the destroyer.

On 30 May 1942, I-16 and I-20 launched midgets outside the British naval base at Diego Suarez, Madegascar.  A third midget, from I-18, failed to launch.  The midget from I-20 damaged battleship HMS Ramillies with one of his torpedoes and sank a tanker with the other.  Neither midget survived, although the crew of the midget from I-20 made it ashore, only to be shot and killed by British soldiers.  An attack on Sydney Harbor the very next evening achieved nothing, and the five midgets employed, including those from I-22 and I-24, were all lost.  I-16, I-20, and I-24 were used to launch midgets off Guadalcanal on 7 November 1942, when the midget from I-20 damaged a transport, but again none of the midgets survived the attack.

None of the C1 Type survived the war.  Famed destroyer escort USS England sank I-16 northeast of the Solomons on 19 May 1944.  I-18 was lost to destroyer USS Fletcher and an aircraft from cruiser USS Helena south of the Solomons on 11 February 1943.  I-20 was an operational loss in the New Hebrides on 10 October 1943, while I-22 was lost operationally in the Solomons on 1 October, 1942.  Patrol Craft PC-487 sank I-24 off Attu in the Aleutians 11 June, 1943.