Our studies showed that family C DNA polymerase (pol III) of the cyanobacterium Synechocystis sp. strain PCC 6803 is phylogenetically close to the Gram-negative dna E group, rather than to the Gram-positive group. However, in contrast to the dna E genes of most of the eubacteria, the cyanobacterial dna E gene has split into two genes, dna E 1 and dna E 2 . The evolutionary origin of the split dna E gene is discussed.