The gene bchG, coding for bacteriochlorophyll a synthase from a variety of green sulfur bacteria and the filamentous anoxygenic phototrophic bacteria, Chloroflexus aurantiacus, Chloronema sp., and Roseiflexus castenholzii HL08, was partially sequenced and compared. The deduced amino acid consensus sequences for green sulfur bacteria and green filamentous anoxygenic phototrophic bacteria were found to belong to the UbiA enzyme family of polyprenyltransferases with the most similar sequences being those of photosynthetic organisms. All deduced amino acid sequences showed a highly conserved region, which includes the motif DRXXD, characteristic of polyprenyltransferases, which was extended to DREVDAINEP for green sulfur bacteria. Neighbor-joining analysis of a protein similitude matrix displayed a relatively high distance between green sulfur bacteria and the other groups. Sequences from green sulfur bacteria were more closely related to those of purple bacteria than to those of filamentous anoxygenic phototrophic bacteria. In addition, internal grouping within green sulfur bacteria was congruent regarding taxonomic features including cell shape, presence of gas vacuoles and NaCl requirement. In addition to bchlG, another gene encoding for a second chlorophyll synthetase, previously tentatively identified as chlG, was also found in Chlorobium tepidum, showing the highest similarities with polyprenyltransferases from chlorophyll- a-containing organisms.