The 2'-5'-oligoadenylate synthetase (OAS) belongs to a nucleotidyl transferase family that includes poly(A) polymerases and CCA-adding enzymes. In mammals and birds, the OAS functions in the interferon system but it is also present in an active form in sponges, which are devoid of the interferon system. In view of these observations, we have pursued the idea that OAS genes could be present in other metazoans and in unicellular organisms as well. We have identified a number of OAS1 genes in annelids, mollusks, a cnidarian, chordates, and unicellular eukaryotes and also found a family of proteins in bacteria that contains the five OAS-specific motifs. This indicates a specific relationship to OAS. The wide distribution of the OAS genes has made it possible to suggest how the OAS1 gene could have evolved from a common ancestor to choanoflagellates and metazoans. Furthermore, we suggest that the OASL may have evolved from an ancestor of cartilaginous fishes, and that the OAS2 and the OAS3 genes evolved from a mammalian ancestor. OAS proteins function in the interferon system in mammals. This system is only found in jawed vertebrates. We therefore suggest that the original function of OAS may differ from its function in the interferon system, and that this original function of OAS is preserved even in OAS genes that code for proteins, which do not have 2'-5'-oligoadenylate synthetase activity.