In the paper are presented the data on revealement of cryptosporidian oocysts (Apicomplexa, Sporozoa) in feces of cattle, swine and sheep of different ages and results of the experimental infection of laboratory animals (rats, mice, rabbits, coypus) with the oocysts detected as well. The latters were attributed to Cryptosporidium parvum species. The analysis of the size characteristic in the isolates of naturally and artificially infected hosts has shown that the oocysts dimensions might vary both in different host species and in different individuals of one host species.