Escherichia coli cells are unable to co-maintain a λdv and a minimal pSC101 replicon. Selection for both plasmids results in a much reduced growth rate of the cells, whereas selection for one plasmid results in a rapid loss of the other. A chromosomal mutant has been isolated, which restores the normal growth rate of cells harbouring both plasmids, but does not alleviate the incompatibility. The nature of this new case of incompatibility is discussed.