10 Bacillus thuringiensis strains were used to determine the susceptibility of Menopon gallinae (L). In order to identify B. thuringiensis from dead insects, previously exposed to bacteria, the streptomycin-resistant mutants of strains were also applied. The experiments were conducted in a group of 20 specimens, in three replications. The absorbtion at 750 nm directly corresponded to the concentration of 23 X 10(6) spores/ml in each assay. The biting lice were exposed on agar plates, inoculated by 0.2 ml of B. thuringiensis preparations dipping for 2, 4 and 8 s or spraying with 0.5 ml of adequate strain. Laboratory trials showed that M. gallinae was susceptible to toxic activity of all tested strains. While the susceptibility to various serotypes varied, depending on the time and mode of infection, 3 subspecies: kurstaki, finitimus and kenyae were usually the most active. The least active strains were the subsp. galleriae and aizawi. The microbiological examination of dead mallophaga treated with B. thuringiensis proved the presence of this bacterium in smears taken from the insects. A comparison of morphological as well as biochemical properties of the antibiotics sensitive and the antibiotics-resistant mutants revealed no difference.