Nucleophaga amoebae belongs to the phylum Rozellomycota (Opisthokonta), a widespread clade of parasites, considered as intermediate link between fungi and microsporidia. This organism is an obligate intranuclear parasite of the free-living amoeba Thecamoeba quadrilineata. The life cycle of this organism is difficult to study, many details require further clarification, and available light-microscopic images are limited in number and quality. We performed real-time observations on the process of parasite propagation in amoeba culture using Eppendorf Cell Imaging Plates and Differential Interference Contrast (DIC) microscopy. Development of the parasite was traced from the engulfment of spores by the amoeba cell to the production of a new generation of spores. Nucleophaga cells proliferate inside the host nucleus. The earliest intranuclear developmental stages that we observed were rounded uninucleate cells located at the margin of the host nucleolus. Growth resulted in formation of a large multinucleate plasmodium, which further became segregated into numerous individual uninucleate sporoblasts. After a period of maturation, sporoblasts transformed into the rounded spores enclosed in the sporophorous vesicle, probably formed by the remnants of the membrane of the plasmodium. At the final stage of the developmental cycle the amoeba cell died, its envelope, as well as the nuclear membrane broke, and the spores were released into the environment. The developmental cycle took approximately 5 days. Infected amoebae never divided, so we can suggest that the infection suppressed mitosis in the host cell.