At 10.20 hours on 11 April 1942, U-154 spotted the unescorted Delvalle (Master Edgar F. Jones) south of Haiti and tried to get into a favorable attack position. The U-boat was spotted by a civilian passenger aircraft off the port quarter of the ship and advised the Delvalle of this discovery, which tried to escape on a nonevasive course at her full speed of 13 knots. At 00.01 hours on 12 April, the submerged U-154 fired two single torpedoes at her but missed as the ship turned towards the U-boat in an attempt to ram it because a lookout had spotted the periscope about 500 yards away. Three minutes later the angry Kölle hastily fired both stern torpedoes but they also missed, the BdU later criticizing him for wasting valuable torpedoes.

At 06.57 hours, the Delvalle was finally hit by a spread of two torpedoes on the starboard side almost simultaneously about 15 feet below the waterline and just forward of amidships. The explosions extensively damaged the ship and destroyed the starboard lifeboats. As the vessel rapidly sank, the launching of the other two boats proved difficult because the severe list. She sank 15 minutes after being hit aft by a coup de grâce at 07.09 hours. U-154 had recieved the distress signals from Delvalle but Kölle believed that he sank a more modern ship of the same name, unknown to him the vessel had been rebuilt before the war with more passenger accommodations. The ship was also claimed by U-203 which misinterpreted her distress signals after an attack on Stanvac Melbourne.

The crew of nine officers, 45 men, five passengers and four armed guards (the ship was armed with one 4in and two .30cal guns) all abandoned ship, with the exception of the ship´s doctor and an able seaman. The boats and rafts stayed together until daybreak, when the motor launch left to reach the coast for help. The launch later reached land at Jacmel, Haiti. The remaining survivors were sighted by a US Navy patrol aircraft the same day and were picked up by HMCS Prince Henry (F 70) (A/Capt J.C.I. Edwards, RCN).