A series of temperature-sensitive mutations affecting maintenance of the F plasmid were mapped by cloning with restriction enzymes. The 14 mutations tested mapped in the region F43.9-46.9 kb, which has been shown to be essential for replication of F. The rates of incorporation of radioactive precursors at the restrictive temperature are consistent with at least some of the mutations primarily affecting plasmid replication rather than partition. Comparison of the curing kinetics of F and mini-F-plasmids showed that the parental F-plasmids were lost more slowly than their mini-F derivatives. This effect is attributed to replication from the secondary replicative origin of F found in EcoRI fragment f7. Mini-F-plasmids containing only F43.9-46.9 kb from wild type F were found to be unstable.