Ribonucleotide reductase (RNR) is central to de novo synthesis of deoxyribonucleotides and essential for all living cells. Three classes have been described; class I is oxygen dependent and represented by two subclasses, Ia (NrdAB) and Ib (NrdEF); class II (NrdJ) is indifferent to oxygen; and class III (NrdDG) is oxygen sensitive. More than one class can be found in an organism, reflecting the oxygen status of its environment. We have investigated, by using PCR and Southern blot, the occurrence of the different classes among species of the gamma -Proteobacteria. Class III an present in all species tested, hut the presence of the other classes varies. Some species contain one unique additional enzyme, class Ia, Ib, or II, whereas others contain two additional enzymes, class Ia and Ib, or class Ia and II.