Representatives of three ancient gene families of the antioxidant enzyme superoxide dismutase (SOD) can be found in most metazoans. In mammals and Caenorhabditis elegans, there is at least one gene each of the cytoplasmic, mitochondrial and extracellular lineages of SOD genes. The cytoplasmic SOD was one of the first enzymes to be implicated in ageing due to its protection against damaging oxygen free radicals. In contrast to other metazoans, insects were thought to lack a gene for the extracellular SOD. We have cloned and sequenced an SOD mRNA in the ant Lasius niger that appears to belong to this extracellular family. Subsequent searches and analyses of SOD gene sequences in insect databases revealed that insects do indeed express all three SOD genes including the extracellular form. We conclude that insects as well as other metazoans appear to have the full repertoire of the three families of SOD.