At 15.52 hours on 27 June 1942 the unescorted and unarmed Polybius (Master Ole P. Stender) was hit by one torpedo from U-128 about 250 miles east of Trinidad, while steaming a nonevasive course at 9 knots. The torpedo had been spotted by a lookout, but it was too late and it struck abaft the #5 hatch directly under the living quarters, blowing off the stern and killing ten crew members. She settled rapidly by the stern and sank within ten minutes. The survivors among the eight officers, 29 crewmen and seven passengers on board abandoned ship in four lifeboats immediately after the hit. The master was questioned by the U-boat before it left the area.
Seven survivors in one boat were picked up the next day by the Dutch steam merchant Dracos and taken to Georgetown, British Guiana. Twelve men in a second boat were picked up after three days by the steam merchant Clarona and brought to Trinidad. The remaining survivors in the other two boats were rescued by an Allied vessel after two days and landed in Trinidad.