The microbial population of the praeputial sac was studied in a group of ten young bulls housed in one section of the rearing farm in the age period from 2 to 14 months when they were transferred to A. I. stations. As to currently occurring microflora, Proteus sp. and Pseudomonas aeruginosa were isolated most frequently. The remaining micro-organisms isolated during the study included E. coli, non-haemolytic and viridizing streptococci, Staphylococcus epidermidis, micrococci, aerobic sporulating bacteria, saprophytic corynebacteria snd gamma-negative non-fermenting rods. Campylobacter foetus subsp. foetus was not detected and the same applies to the other species of this genus (Campylobacter sp.). When the bullocks were transferred to the rearing farm, the mycoplasms were no longer isolated from the praeputial mucous membrane. Mycoplasms were determined in all the studied animals after six months' stay in group. The isolated strains were identified as M. bovigenitalium by the epiimmunofluorescence method.