1. A review of the protozoa reported from animal-parasitic helminths is first made. 2. A detailed, description is given of the morphology of the microsporidian found in Moniezias in this country. 3. This parasite has been shown to be identical with Nosema helminthorum Moniez, 1887, Plistonhora helminthophthora Labbe, 1899, and Nosema bischoffi Weiser, 1951. The valid name for this organism therefore is Nosema helminthorum Moniez , 1887. 4. A 'pre- metachromatic granule' appearing in the sporont stages, and the metachromatic bodies of the mature spores have been studied from the point of view of their chemical nature. 5. The spore wall of this parasite has been proved to consist of chitin. 6. The mode of emergence of the sporoplasm has been shown to occur in a manner hitherto unobserved in microsporidia. 7. N. helminthorum has been shown to infect Hymenolepisnana of mice and rats, under experimental conditions. In three tapeworms, this microsporidian undergoes a similar development to that in the Moniezias, but the spores assume a smaller size. 8. A Taenia aaginata of man was also infected with this organism but only early division stages were obtained. 9. Two oribatid mites Xebillus tegeocranus and Ceratoppia bipilis were fed on the spores of N. helminthorum, and the cells of the midguts of these mites became infected. The spores appearing in the mites were smaller than those in the mice worms. 10. The significance of the experinmental infections has been discussed, and the life-cycle of N. helminthorum has been described from them. 11. The host distribution and the geographical distribution of N. helminthorum and its host-parasite relationships have been considered. This parasite is shown to be non-pathogeneic and the question of its host-restriction has been discussed. 12,. A paraneoxenous association between Giardia ovis and Nematodirus fillicollis of sheep has been discussed. 13. Finally a description is given of a new genus of schizolregarine and a new species of microsporidian, from the fat body of the flour beetle Tribolium castaneum. The oöcysts of the schizogregarine were used in some of the experimental infections of Hymenolepis nana.