SUMMARY: Escherichia coli, C-M7, a His+Nif+ hybrid obtained by intergeneric mating with a Klebsiella pneumoniae donor strain, also inherited the unselected markers gnd and rfb. The R factor, R144 drd 3, which had been used to confer fertility on the donor, was present, detectable as covalently closed circular DNA of molecular weight 69 X 106 daltons. No other species of supercoiled DNA were isolated and the elimination of R144 drd 3 did not result in the loss of Klebsiella genes. Segregation analysis of donor markers indicated that the Klebsiella DNA was integrated at the his region of the E. coli chromosome in the probable order his-gnd-nif-rfb. Strain C-M7 produced a nitrogenase physiologically identical to that of K. pneumoniae, but synthesized a heteromeric species of gluconate-6-phosphate dehydrogenase.