A stable mixed bacterial culture was obtained by chemostat enrichment using dimethyl-sulphoxide as a carbon and energy source. This culture could not only rapidly oxidize dimethyl-sulphoxide but also dimethyl-sulphide. Enzyme determinations indicated that an important part of it consisted of methylotrophs, which assimilated carbon via the serine pathway. Indeed plate counts revealed the majority of the community to be a Hyphomicrobium species. This organism, designated Hyphomicrobium EG, is an obligate methylotroph which can only grow aerobically on several different C1-compounds. Its performance on dimethyl-sulphoxide was compared with that of the community and of another recently isolated strain, Hyphomicrobium S. The mixed culture, Hyphomicrobium EG and Hyphomicrobium S had a mu max of 0.08, 0.08 and 0.014 h-1 respectively. The KS for dimethyl-sulphoxide was the same for all three cultures (3-6 microM), whereas that for dimethyl-sulphide of Hyphomicrobium EG after growth on dimethyl-sulphoxide was 3-fold higher than that of the other two cultures (48 and 16 microM respectively). After growth on dimethyl-sulphide it improved to 3 microM. Dimethyl-sulphide respiration was maximal at a concentration of 100 microM; higher concentrations were inhibitory. One of the accompanying organisms, a pink methylotroph, was able to derive energy from the oxidation of thiosulphate. Available cultures of Thiobacillus MS1 that were reported to be able to utilize dimethyl-sulphide could no longer metabolize this compound.