The incidence of bacteriocinogeny and lysogeny was followed in bacteria of 3 recently acknowledged species of the genus Escherichia: E. hermanii, E. vulneris and E. fergusonii. Almost all of the strains examined were of human origin. In 30 strains of E. hermanii no one was found bacteriocinogenic while 57% were lysogenic, in 30 strains of E. vulneris none was found to be bacteriocinogenic and only 10% were lysogenic, and in 50 strains E. fergusonii 12% were bacteriocinogenic and 40% lysogenic. From the 6 bacteriocinogenic strains of E. fergusonii, 3 were producers of colicin E1, 2 of colicin Ib and 1 of colicin Ia. In addition, 3 E. fergusonii strains produced aerobactin.