Five nursing opossums (Didelphis virginiana) were each inoculated with 5 x 10(6) Cryptosporidium parvum oocysts of calf origin. Following inoculation, endogenous stages of C. parvum were observed in the ileum, cecum, and colon of these opossums. Two of three noninoculated pouch mates acquired infections during the study based on examinations of feces and tissue sections of all eight opossums. Mild diarrhea was observed in four of seven opossums harboring C. parvum, although none died as a result of the infection. Under the conditions of this study, C. parvum appeared to be only mildly pathogenic for opossums.