A cDNA designated MOv2 was isolated from anOnchocerca volvuluslibrary on the basis of its product's recognition by an antiserum raised against the infective stage. Immunogold electron microscopy revealed a high density of antigenic sites associated with the annulae of the L3 cuticle and with the uterine wall of the adult female: a general, low density of labell ing occurred in all developmental forms. Western blotting confirmed the presence of the antigen throughout the life cycle and the existence of an immunologically cross‐reactive homologue in the related filaria,Acanthocheilonema viteae. The antigen was shown to be secreted by infective larvae and adult females ofA. viteae. Sequence comparisons revealed two homologues of MOv2 (Ov‐20, Ov‐9) which had been selected independently by other laboratories on the basis of their specific recognition by human onchocerciasis infection sera. The IgG antibody response against MOv2 in cattle experimentally infected withO. lienalisrevealed the induction of a response during the prepatent period that was strongly boosted at the onset of patency. However, only a proportion of infected cattle responded with a detectable level of anti‐MOv2 antibodies. The appearance of MOv2 in larval cuticle and secretions prompted us to evaluate it as a candidate molecule for prophylactic immunization. Trials performed in theA. viteae/Mongolian jird model of filariasis revealed that recombinant MOv2 induced a host‐protective response, reducing worm recoveries by 36–55% following a challeng