A total of 580 fecal samples from 30 cows and their calves from three dairy farms in São Paulo State, Brazil, was collected monthly over a period of 1 year (April 1994 to March 1995). A combination of water-ether centrifugal sedimentation technique followed by sucrose centrifugal flotation was used for neonatal samples and sucrose centrifugal flotation for the remainder of the samples. Cryptosporidium muris was found in calves from all farms, although the parasite was not detected in the dams. C. muris oocysts were excreted by 14 of the 30 calves, on at least one occasion during the year. Of the 307 calf fecal samples examined, 53 (17.3%) were positive for C. muris. The mean persistency of oocyst shedding in feces was 4 +/- 3 months. Sixty-four per cent of the positive calves excreted few oocysts, while 36% discharged many oocysts for a longer period (P < 0.05). Oocysts were first detected at the mean age of 3 +/- 1 months. The age of the positive calves ranged from 6 weeks to 14 months.