The third patrol of the U-128 was her most productive, Kapitänleutnant Ulrich Heyse, sank five ships totaling over 35,000 tons. The last one was the twenty-three year old freighter Polybius on June 27, 1942. Ten minuets after being hit by a single torpedo, she sank stern first into the Caribbean Sea. Thirty-four survivors made it into the lifeboats, but sadly ten men were killed by the explosion. Within three days everyone had been rescued by several passing ships.

The U-128 was sunk less than a year later on May 17, 1943, but by then she had a new commanding officer, Kapitänleutnant Hermann Steinert, it was his only command. Ulrich Heyse was later awarded the Knights Cross and promoted to Korvettenkapitän. He made two more patrols with U-128, but never had another U-boat command, he survived the war and died in 1970.