Recently, a new species of eukaryotic endoparasites of amoebae was described as Nucleophaga striatae, clustering within the previously established Rozellomycota on the basis of DNA sequence data. As a supplement to the molecular phylogenetic identification, this paper focuses on the EM description of this new species in the experimental host Thecamoeba foliovenanda. The spores of Nucleophaga are engulfed by the amoeba by phagocytosis, and then the endoparasites reach the nucleus of the amoeba, where one of the first detectable changes is the enlargement of the circularly arranged pieces of the nucleolar substance. Subsequently, the parasites grow and develop into multinucleate plasmodia, causing a 2-3-fold enlargement of the host amoeba's nucleus. As a result of multiple nuclear divisions within the plasmodia (endogenous sporogony), sporoblasts are forming which contain a network of endoplasmic reticulum and mitochondria. The sporoblasts, which more and more resemble spheres, form a wall and mature by becoming spores. These nearly spherical spores have diameters of 1.4 µm to 1.5 µm and a thick cell wall of ~50 nm. Other free-living amoebae such as Saccamoeba and Vannella (Amoebozoa), and the more distant Naegleria and Willaertia (Heterolobosea, class within phylum Percolazoa), have proven to be resistant to infection, showing that these intranuclear parasites are infectious only for members of the genus Thecamoeba (Amoebozoa). [ABSTRACT FROM AUTHOR]