DNA of 96 strains of the genera Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, and Nitrosovibrio was isolated and analysed spectrophotometrically. Percentages of guanine plus cytosine (G + C) content, genome sizes, and DNA-DNA homologies were determined. The results indicated the presence of eight Nitrosomonas species, three or four Nitrosococcus species, five Nitrosospira species, and two species of both Nitrosolobus and Nitrosovibrio. DNA homologies between strains of a separate species ranged from 56-100%. Average homologies between strains of different species were 33% in Nitrosococcus, 36% in Nitrosomonas, 37% in Nitrosolobus, 40% in Nitrosospira, and 42% in Nitrosovibrio. Average homologies between species of different genera were 33% and thus not significantly above the background value of 30% detected between DNA of ammonia-oxidizing bacteria and Escherichia coli. Genome sizes ranged from 1.90 - 2.74 X 10(9) dalton in Nitrosomonas, 2.09 - 2.37 X 10(9) dalton in Nitrosococcus, 1.87 - 2.15 X 10(9) dalton in Nitrosospira, 1.92 - 2.10 X 10(9) dalton in Nitrosolobus, and 1.91 - 2.15 X 10(9) dalton in Nitrosovibrio. Differences in genome sizes were in accordance with DNA homologies.