Abstract: This paper deals with the ultrastructure of the trophozoite of Myxobolus drjagini, the pathogenic agent for the whirling disease of Hypophthalmichthys molitrix.A large number of active trophozoites were taken from the cavity of the cranium of the diseased fish. They were then doubly fixed, embedded in Epon 812, sectioned and stained.Under electron microscope can be seen the plasmodial membrane of the trophozoite which is continuous with numerous pinocytic channels in the ectoplasm. The pinocytic channels are branched intp various lengths. The nucleus is delimited by two unit membranes with several nuclear pores. Interior to the zone of pinocytic channels is an outer zone of endoplasm rich in mitochondria. In the cytoplasm near the nucleus can be seen the Golgi apparatus, developed endoplasmic reticulum, microfibrils, microtubules, and an array of lipoid granules of unequal sizes. Functions of the lipoid granules are not known. The centrioles are not seen.