This study describes the complete development (from sporozoites to sporulated oocysts) of Cryptosporidium parvum (human and cattle genotypes) in the HCT-8 cell line. Furthermore, for the first time the complete life cycle was perpetuated in vitro for up to 25 days by subculturing. The long-term maintenance of the developmental cycle of the parasite in vitro appeared to be due to the initiation of the auto-reinfection cycle of C. parvum. This auto-reinfection is characterised by the production and excystation of new invasive sporozoites from thin-walled oocysts, with subsequent maintenance of the complete life cycle in vitro. In addition, thin-walled oocysts of the cattle genotype were infective for ARC/Swiss mice but similar oocysts of the human genotype were not. This culture system will provide a model for propagation of the complete life cycle of C. parvum in vitro.