Various microbial eukaryotes have been reported as endocytobionts in a wide range of protists, primarily as parasites. Those which parasitize the nucleus (Nucleophaga-like) or the cytoplasm (Sphaerita-like) of free-living amoebae, described since the end of the 19th century, have been the subject of recent studies by our team, revealing that they belong to the same group, the Rozellomycota, linked to fungi and ancestors of Microsporidia. We report here the observation of an amoeba isolated from a sycamore tree near Koblenz, Germany, and found to be infected by an intranuclear parasite. By light microscopy, the host amoeba was identified as Thecamoeba striata while the intranuclear parasite looks like previously studied Nucleophaga species. The parasite proliferates in the enlarged nucleus of the host amoeba filling it with its spores, ultimately causing its death by rupture of the membranes. Compared to other Nucleophaga spp., the current parasite grows more slowly. The 18S rDNA sequence of the parasite was thus obtained to perform biomolecular analyses. Phylogenetic analysis confirms that the parasite belongs to Nucleophaga, differing from the other described species in sequence similarity values. We propose to consider this strain as a new species, Nucleophaga striatae sp. nov.