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 D N A -D N A 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.
D N A homologies between strains of a separate species ranged from 5 6 -1 0 0 % . 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 โข 1 0 9 dalton in Nitrosomonas, 2.09-2.37โข dalton in Nitrosococcus, 1.87-2.15x109 dalton in Nitrosospira, 1.92-2.10โข dalton in Nitrosolobus, and 1.91-2.15 โข 109 dalton in Nitrosovibrio. Differences in genome sizes were in accordance with D N A homologies.