In an attempt to obtain the continuous development of Hammondia hammondi in vitro, culture of the parasite was performed in 3 cell lines. Although organisms were present in culture fluids of feline kidney cells (CRFK) for as long as 3 mo, continuous culture was not possible. However, for the first time, cysts of H. hammondi were observed in cell culture from day 6 after inoculation of sporozoites. Ultrastructure of the H. hammondi cysts was similar to that observed for in vitro-obtained Toxoplasma gondii cysts. Feeding a cat with these in vitro-developed cysts resulted in oocyst shedding 5 days after ingestion.