The accuracy of replication and transfer of a lacI gene on an F' plasmid was measured. Following conjugal transfer of the F', a small but reproducible increase (1.8-fold) in the frequency of lacI  ^- mutations was detected. Among these, however, the frequency of nonsense mutations was 15-fold higher than in the absence of transfer. This corresponds to a 300-fold increase in the rate of base substitutions per round of replication compared with normal vegetative DNA replication. The amber mutational spectra revealed that, following conjugal transfer, mutation frequencies were increased markedly at all sites detected. In addition, an increase in G:C → A:T transitions was noted and was due almost entirely to an enhanced proportion of mutants recovered at the spontaneous hotspots (amber sites 6, 15 and 34). recA-dependent processes were not responsible for the increase in mutation, since similar results were observed with various recA  ^- donor and recipient combinations. These results demonstrate that the fidelity of conjugal DNA replication is considerably lower than that of vegetative DNA replication.