BETWEEN February and June 1947, 20·5 per cent of the males, 3·7 per cent of the females and 0·3 per cent of the copepodids of Pseudocalanus elongatus Boeck, collected from six stations between Flamborough Head and the Dogger Bank, were found to be parasitized by Blastodinium contortum hyalinum Chatton. Almost all the observed parasites were found to be in the tetrablastic or a later stage, with a length of 440—560µ. They occurred singly in the vast majority of cases, but very rarely two individuals were found in the same Pseudocalanus. As the material was unstained, it is likely that most of the earlier stages of the parasite were overlooked, particularly in the younger stages of the copepodids, and that the proportion of infected individuals was really higher than that given.