The primary structure of an immunodominant antigen of the filarial parasite, Onchocerca volvulus was deduced from cDNA sequence analysis. Using affinity-purified antibody from onchocerciasis patients from West Africa, we have isolated a cDNA clone from a lambda gt11 cDNA expression library derived from microfilariae-producing female O. volvulus. The open reading frame encodes 152 amino acids, and the deduced sequence predicts a Mr of 16,850 (consistent with the apparent Mr of 18,000 of the immunoprecipitated in vitro translated product). The primary translation product contains a putative signal peptide of 16 amino acids. The mRNA coding for this antigen has an estimated size of 950 nucleotides. Furthermore, immunoelectron microscopy established that the antigen encoded by this clone is present in the hypodermis, the cuticle, and in the uterus of the filarial worms. Since this antigen is recognized exclusively by sera from onchocerciasis patients, and not by other sera from patients infected by other filarial parasites, it may prove to be an especially valuable tool for improving the specific diagnosis of onchocerciasis.