At 1000 hours, Pietro Calvi observed a dark vessel at a distance of 2,000 metres. At 1013 hours, the distance had closed to 600 metres when C.C. Caridi ordered tube no.2 to be fired. Due to a malfunction of the interphone, both torpedoes that had been readied (one 533mm and one 450mm) were fired. They both missed, but were observed by the target who opened fire with a machine gun and sent an SOS. This was the British Carlton (5,162 GRT, built 1924) from convoy OB.260d.

At 1448 hours the submarine turned for a stern shot (450mm) at the same target from a distance of 1,000 metres but the torpedo missed ahead.

At 1445 hours a second stern shot (533mm) was fired from a distance of 1,300 metres. It squarely hit the target after 62 seconds, and Carlton (5162 GRT, built 1924) sank in six minutes. The submarine found 30 survivors in two boats and asked them if they needed anything (the captain asked for cigarettes). Thirty-one were lost, and four rescued. According to C.C. Caridi the vessel's SOS gave a position 30 miles to the north of the actual position.