Copper-zinc superoxide dismutases ([Cu,Zn]-SODs) are ubiquitous in eukaryotes but have rarely been found in prokaryotes. A gene for [Cu,Zn]-SOD (sodC) has recently been cloned from Haemophilus influenzae type b and H. parainfluenzae, so other Haemophilus and related species were screened for the presence of [Cu,Zn]-SODs by visualization of bands of SOD activity in non-denaturing polyacrylamide gels and by gene probing. Strains of H. aphrophilus, H. paraphrophilus, H. haemolyticus, H. paraphrohaemolyticus, some non-typable H. influenzae, H. haemoglobinophilus (canis) and H. parasuis were all found to have [Cu,Zn]-SOD activity (inhibited by 2 mM-cyanide) in polyacrylamide gels. In a Southern blot analysis, DNA from H. aphrophilus, H. paraphrophilus, H. haemolyticus and [Cu,Zn]-SOD-containing non-typable H. influenzae--but not the other species--hybridized to a 360 nucleotide DNA probe containing the 5'-part of sodC cloned from H. influenzae type b. Bacterial [Cu,Zn]-SODs are more prevalent than has previously been recognized.