Sommerstorffia spinosa Amaudow (Saprolegniales, Oomycetes) infects loricate rotifers by both endoparasitic and predacious means, using sporelings and pegs, respectively. The sporeling is a lecythiformshaped structure derived from the encysted, secondary zoospore, whereas the peg is a narrow terminal protuberance of a short, hyphal branch. In electron microscopy of thin sections, however, infective organs are very similar to each other, being packed with many, large (approx. 1.0 Atm diam), electrondense vesicles in their apical portion. When that portion is engulfed by rotifers, both infective organs secrete an amorphous, electron-dense, adhesive mass containing a number of bubbles. Sommerstorffia spinosa Arnaudow, a watermold parasitic on small loricate rotifers, has been reported from Bulgaria (Arnaudow, 1923), several places in the U.S.A. (Karling, 1952; Sparrow, 1929) and England (Prowse, 1953). In the present study, we obtained the fungus species in Japan and used it as material for electron microscopic observations. It is of interest that the fungus has two different manners of infection, endoparasitic and predacious, by different infective organs respectively. In the former case, the fungus infects the animals with sporelings. The sporelings are lecythiformor flask-shaped structures with narrow necks which are always terminated with globose knobs. Infection of rotifers is initiated by the animals engulfing the globose knob of sporelings. After penetration from the knob, an infection thallus soon develops inside the body of the rotifer and it eventually consumes the body content. Soon after the endozoic thallus forms, the fungus also develops two (or rarely only one) short, hyphal branches from the basal part of the sporeling external to the infected rotifer. Each of the two hyphal branches terminates with narrow protuberances called pegs with which other, healthy, animals are captured by the predacious manner of infection. The fungus then further develops its hyphal system by consumption of the captured rotifers. In this paper, we show transmission electron micrographs from ultrathin sections of these two different kinds of parasitic structures.