Rozella allomycis is an obligate endoparasite of the water mold Allomyces and a member of a clade (= Opisthosporidia) sister to the traditional Fungi. Gaining insights into Rozella's development as a phylogenetically pivotal endoparasite can aid our understanding of structural adaptations and evolution of the Opisthosporidia clade, especially within the context of genomic information. The purpose of this study is to characterize the interface between R. allomycis and Allomyces anomalus. Electron microscopy of developing plasmodia of R. allomycis in host hyphae shows that the interface consists of three-membrane layers, interpreted as the parasite's plasma membrane (inner one layer) and a host cisterna (outer two layers). As sporangial and resting spore plasmodia develop, host mitochondria typically cluster at the surface of the parasite and eventually align parallel to the three-membrane layered interface. The parasite's mitochondria have only a few cristae and the mitochondrial matrix is sparse, clearly distinguishing parasite mitochondria from those of the host. Consistent with the expected organellar topology if the parasite plasmodia phagocytize host cytoplasm, phagocytic vacuoles are at first bounded by three-membrane layers with host-type mitochondria lining the inner membrane. Thus, Rozella's nutrition, at least in part, is phagotrophic in contrast to osmotrophic nutrition of traditional fungi.