The influence of cryptosporidial abomasitis on digestive anaerobic microflora in feedlot cat¬tle with spontaneous Cryptosporidium andersoni colonization of abomasum was shown. Significant differences were found after the cultivation of abomasal content. Scopulariopsts brevieaulis was detected in damaged areas of infected abomasum. PCR analysis of ruminai fluid showed no differences between the two animal groups regarding qualitative composition of anaerobic and facultative anaerobic rumen microflora. The con¬centration of volatile fatty acids (acetate, propionate, butyrate) and ammonia in the rumen content showed that examined metabolic parameters were within normal limits except for ammonia content, which was higher in infected (291 mg/kg) than in healthy animals (203 mg/kg). Cryptosporidium andersoni LINDSAY et at. (2000) colonizes the abomasum of up to 10 % of older beef and dairy cattle. This organism has only recently been identified in feedlot calves and dairy cows. The first cases of cryptosporidial infection of abomasum in Czechia were reported by Pavlásek (1994). This parasite has been shown to infect cattle for several years. Functionally, the infection leads to impairment of protein digestion by increasing gastric pH and inhibition of the proteolytic function of pepsin (Anderson et ah 1990).