SUMMARY: The base composition of purified DNA from 28 strains of acetic acid bacteria was determined. Most strains of the genus Gluconobacter clustered closely together at 60.6–63.4 % (guanine + cytosine) of total base. All strains of the Acetobacter aceti biotype lay within the range 55.4–64.0 % (guanine + cytosine). The close relationship and possible common phylo-genetic origin of the genera Gluconobacter and Acetobacter is again stressed by these results. The base composition of DNA from acetic acid bacteria and from species of Pseudomonas was very similar, confirming the suspected close relationship between these groups. There is a noticeable agreement between the sequences of Acetobacter strains, arranged according to increasing % (guanine + cytosine) and arranged according to increasing enzymic equipment: strains with greater biochemical activity have on the whole also a higher % (guanine + cytosine) in DNA. The range of the compositional distribution of DNA molecules is on the whole broader in Acetobacter than in Gluconobacter. The results corroborate previous conclusions that both biotypes contain clusters of strains without species differentiation. A comparison of the paper chromatographic analysis with the method of thermal denaturation (‘melting point’) for estimating base composition of DNA showed that the latter method was to be preferred in routine analysis because of its ease, rapidity and reproducibility.