An essential part of the chromosome replication origin of E. coli K-12 and B/r was replaced by the plasmid pOU71. The average initiation mass of replication for pOU71 decreases with increasing temperature. The constructed strains were grown exponentially at different temperatures, and cell sizes and DNA content were measured by flow cytometry. The average DNA content increased with increasing temperature, but the cell size distribution was largely unaffected. Furthermore, cells in which DNA replication had not yet initiated (cells in the B period) became less abundant with increasing temperature. The increased DNA content could not be explained by an increase in the length of the C period. It is concluded that chromosome replication does not trigger cell division in E. coli, but that the chromosome replication and cell division cycles of E. coli run in parallel independently of each other.