Abstract
Colicin-producing plasmid-containing cells of E. coli exhibit frequency-dependent selection when grown in glucose-limited continuous culture with the corresponding plasmid-free strain. The bases of this frequency-dependent effect are shown to be (1) the lower growth rate of the plasmid-containing strain under these conditions, and (2) the production of colicin, which attenuates the growth rate of the plasmid-free strain. These results are discussed in relationship to the maintenance of genetic variation in prokaryotes.