Nosema sp. Cort, Hussey & Ameel, 1960, previously reported as a hyperparasite of strigeoid trematode larvae and apparently specific for strigeoids, is described as a new species, N. strigeoideae, with fresh spores averaging 4.7 by 3.1 u. Primary infection appears to be in the wall of the trematode sporocyst, spreading from there to the developing embryos.