30 May 1942. On 30 May 1942, I-18′s midget submarine suffered engine failure and could not launch, but I-16 and I-20 launched their midget submarines off Diego-Suarez. I-20′s midget torpedoed Ramillies at 20:25. At 21:20, while British corvettes dropped depth charges, I-20′s midget torpedoed and sank the 6,993-ton tanker British Loyalty in shallow water. After the midget ran aground, its two-man crew reached shore and attempted to make their way overland to the designated recovery area, but they were reported to the British as having been seen outside Anjiabe at around 11:00 on 1 June and died in a gunfight with Royal Marine Commando No. 5 on 2 June 1942 at Amponkarana Bay (). One Royal Marine also died in the exchange of gunfire. Ramillies survived the attack and departed for Durban ten days later, although the Japanese assessed her as sunk after I-10′s floatplane noted her missing from Diego-Saurez during a reconnaissance flight. British Loyalty later was refloated, towed to Addu Atoll, and scuttled. On 3 June 1942, after I-16 and I-18 had departed the recovery area, I-20 arrived and made an unsuccessful attempt to contact the midget submarines and their crews. She then joined the rest of the "A" detachment in anti-shipping operations. She torpedoed and sank the Panamanian 5,086-ton armed merchant ship Johnstown at on 5 June, the Greek 5,209-ton merchant ship Christos Markettos at on 8 June, and the British 7,926-ton armed merchant ship Mahronda at on 11 June 1942. On 12 June, she sank the Panamanian 2,052-ton merchant ship Hellenic Trader at with gunfire and torpedoed and sank the British 5,063-ton merchant ship Clifton Hall at . After refueling from Aikoku Maru on 19 June 1942, I-20 torpedoed the Norwegian 5,063-ton armed merchant ship Goviken, which was on a voyage from Aden to Lourenço Marques in Portuguese East Africa, on 29 June 1942. Goviken sank less than 20 minutes later at . On 30 June, I-20 attacked the British 5,311-ton armed steam tanker with gunfire at . She fired fifteen rounds and scored one hit. After Steaua Romana returned fire, I-20 submerged and fired a torpedo, but it exploded prematurely. Steaua Romana dropped a smoke float in attempt to conceal herself and tried to escape, but I-20 fired a second torpedo which sank her. That evening, I-20 was able to identify her victim as Steaua Romana by intercepting Allied radio communications. Concerned by a number