CO2-reductive acetogenic bacteria are ubiquitous in anaerobic habitats and are physiologically and phylogenetically diverse. The latter characteristics have rendered their diversity in natural environments, their distributions, and their ecological function(s) difficult to assess. Recently introduced polymerase chain reaction (PCR) primers for specific amplification of the structural gene encoding formyltetrahydrofolate synthetase (FTHFS, EC 6.3.4.3), a key enzyme in the acetyl-CoA pathway of acetogenesis, have facilitated studies of acetogen diversity and ecology. These primers amplify an approximately 1100-bp segment of the FTHFS gene. FTHFS sequences have been recovered from authentic acetogens, from sulfate reducing bacteria, and from a variety of other nonacetogenic bacteria. Phylogenetic analyses segregated these sequences into distinct clusters, only one of which contained sequences from known acetogens. This chapter describes the PCR primers, defines conditions for successful amplification of FTHFS sequences, and details the phylogenetic analysis of the FTHFS sequences. Information on the types of sequences that have been recovered from natural acetogen habitats and how they have been interpreted is also included.