The influence of L-serine on the growth of different strains of the genus Bacillus was investigated. It has been observed that the addition of L-serine to minimal synthetic media results in an inhibition in the growth of certain strains of Bacillus spp. but not B. thuringiensis. Then L-serine-resistance phenomenon was used in isolation of B. thuringiensis strains from soil. An isolation method with media supplied with L-serine was compared to the previously applied procedure (isolation on nutrient agar). L-serine-selective medium appeared to be more effective in isolation of Bt strains.