DURING the course of an investigation into oogenesis in the ascidian Molgula manhattensis a protozoan was discovered parasitizing a few of the nearly mature and fully mature eggs of one gonad. The ascidian which contained the infected gonad was one of several of the same species taken from Plymouth Sound in June 1963. Although there was insufficient material to classify the parasite accurately, the similarity of the observed stages to certain stages in the life-cycles of the Haplosporidia suggests that it is allied to this group1,2.