Studies on cryptosporidiosis and giardiosis were carried out between March and April 1997 on 75 calves from 9 selected farms of Wielkopolska macroregion. Faecal specimens from calves, 3-13 days old, were screened for oocysts of C. parvum using Ziehl-Neelsen staining and both for oocysts of C. parvum and cysts of Giardia sp. using direct immunofluorescent (MerIFluor Cryptosporidium/Giardia) assay. The oocysts of C. parvum assessed by Ziehl-Neelsen staining were revealed on 6 (67%) of 9 farms examined. The prevalence of infection ranged from 20-88%, and in some farms intensity of oocyst shedding was very high. However, in 35 calves assessed for mixed infections of C. parvum and Giardia sp., oocysts of Cryptosporidium were found in 18 (51%) calves and cysts of Giardia sp. were detected in 5 (14%) of 35 calves. Only in one calf was found coinfection with both parasites. The intensity of Giardia sp. infection was extremely low. Histological examination of the gut sections from immunosuppressed BALB/c mice experimentally infected with C. parvum isolates from calves revealed endogenous stages of C. parvum on the brush border of the ileum. The high prevalence and intensive shedding of cryptosporidial oocysts by calves in farms examined in this study suggests that naturally infected calves may be significant reservoirs for C. parvum infections in man and wild animals.