ColA-CA31 plasmid DNA region, responsible for the autonomous replication, has been sequenced. The replicative regions of plasmids ColA and ColE1 share the high homology demonstrated in the paper. DNA nucleotide sequences necessary for initiation of RNA I and primer RNA transcription were localized on the ColA genome. RNA I and preprimer RNA of ColA and ColE1 have the similar secondary structure including three palindromes. The absence of homology in ColA and ColE1 hair-pin loops suggests the compatibility of these two plasmids.