The absence of a self-sustaining in vitro propagation method for Cryptosporidium parvum is a major obstacle for research on this parasite. Conventional cell monolayers are unsuitable for long-term parasite propagation because the level of infection decreases over time and few oocysts, if any, are produced. The interaction between parasite and host cell was studied to identify factors limiting parasite development in vitro. Loss of substrate adherence and death of parasitized host cells was observed in 2 epithelial cell lines. Nuclear morphology, DNA laddering, annexin V binding, and terminal deoxytransferase-mediated dUTP nick end labeling indicated that host cell death occurred by apoptosis. At 6 hr postinfection, only a minority of infected cells remained in the monolayer, and few survived the initial phase of parasite development without losing adherence. Treatment of infected monolayers with caspase inhibitors drastically reduced cell detachment but failed to increase the number of parasites in monolayers. In contrast, cell cultures grown on laminin-coated plates showed a higher proportion of infected cells. These observations indicate that cell detachment and apoptosis in C. parvum-infected cell culture negatively affect parasite survival in vitro.