The minimum region required for the replication of the marine Rhodabacterendogenous plasmid pRD31 has been sequenced. This region is located on a 1367-bp HincII-PstI restriction fragment and is 62% rich in G-C base pairs. A region homologous to the bacteriophage P1 repA promoter, which overlaps two inverted repeats, has been identified. Plasmids with mutations in this 1367-bp region could not replicate in marine Rhodobacterhosts. This is the first identified replication origin of a photosynthetic bacterium.