The dnaAcos mutations are phenotypic suppressors of dnaAts46 that are co-transduced with dnaA, render the cell cold sensitive, and cause an excess of chromosome replication relative to cell mass when the cells are shifted from 42 °C to 32 °C. We have used pulse labelling and DNA-DNA hybridization to follow the effect of a temperature shift on the replication of the chromosome and of the plasmids pSC101, RTF-Tc, and λdv in such strains. After a shift of a dnaAcos strain from 42 °C to 32 °C (non-permissive temperature), initiation of the chromosome and replication of the plasmid pSC101 are stimulated, while the dnaA-independent plasmid RTF-Tc is not affected. The presence of pSC101 does not affect the level of overinitiation of the chromosome. The presence of λdv suppresses the cold sensitivity of dnaAcos mutants and allows the cells to grow at both 32 °C and 42 °C. The presence of λdv suppresses the overinitiation of chromosome and of pSC101 replication at 32 °C. Previous reports had shown that these suppressions involve an interaction between the dnaA product and the λP protein, which is also known to interact with dnaB. We show here that the mutant prophage P1 bac-crr, which produces high levels of a dnaB analogue, suppresses the dnaAcos phenotype, while wild type P1 does not. These results suggest that initiation involves interactions between the dnaA and dnaB products.