A nourseothricin-producing Streptomyces noursei strain was continuously cultivated in a chemostat equipped with a stirrer for mechanical fractionation of the mycelium. Different cultivation conditions allowed the selection of six types of differentiation mutants after the culture had reached a population genetically stationary state. The mutants showed an altered control pattern of sporulation as well as altered antibiotic biosynthesis and antibiotic resistance. In addition, the stability of the recombinant plasmid pIJ385 in several differentiation type mutants as host strains was tested. The results suggest that there exists a strong correlation between the cultivation conditions employed and the type of differentiation mutants selected.