Cloning of specific regions of RK2, a broad host range incompatibility group P plasmid, has revealed three genes: kilA, kilB, and kilC. Each of these genes can cause loss of viability of an Escherichia coli host. This effect on the host is normally prevented by the functions of three additional RK2 genes: korA, korB, and korC. Each kor gene is specific for a particular kil gene. The kil and kor genes are located in four distinct regions of the RK2 genome. The three kil genes are not clustered and, with the possible exception of kilA, they are also well separated from their corresponding kor genes. We have found that the korA and korB determinants are not peculiar to RK2 but instead are highly conserved throughout the incompatibility group P plasmids.