A new genus and species of parasitic copepod, Megallecto thirioti, has been found in a horizontal plankton haul at 100 m depth in the eastern north Atlantic. Its host is very probably a pteropod mollusc belonging to the thecosome group. This copepod would appear to be related to Micrallecto uncinata Stock, 1971 and Nannallecto fusii Stock, 1973, but differs from both in its much larger size and in several important structural features.