The genomic GC contents of both nitrogen-fixing and non-fixing members of eight genera of bacteria are investigated. Analysis by t-tests showed that in the two aerobic genera investigated (Aquaspirillum and Vibrio) there is a significantly higher GC content in the nitrogen-fixing members of the genus than in those unable to Fix nitrogen, whilst in anaerobic genera there is either no GC bias, or in the case of two genera (Rhodospirillum and Clostridium) there is a significantly higher GC content in the non-fixing organisms. This suggests that, in many genera, directional mutational pressures are different in nitrogen-fixing and non-fixing organisms. These results are discussed in the light of known mechanisms of mutation pressure and their relation to environmental variables.