To investigate the prevalence of Cryptosporidium infection in horses, a total of 200 fecal specimens were collected randomly from five farms in Tehran. Samples were taken during a year, 50 specimens each season. Of these specimens, 57 were obtained from Caspian Pony. The presence of oocysts in the samples was confirmed by modified Ziehl-Neelsen staining of direct smears of the fecal material from each horse. The results show a significant relationship between infection and season, so that the most infected cases were seen in summer. According to the results, the highest parasite activity is seen in warm weather, so care must be taken to prevent its transmission to human.