We have previously described the major yolk protein, vitellin, in the British blackfly Simulium ornatum Meigen. Here we demonstrate that vitellogenin, synthesized in the fat body and secreted into the haemolymph, is composed of subunits with the same approximate molecular weight as vitellin, namely 200 and 68 kDa. Simulium ornatum is the natural vector for the cattle filarial nematode Onchocerca lienalis Stiles, which induces host fecundity depletion. A significant reduction in ovarian vitellin content was associated with infection by intrathoracic injection of 20 O. lienalis microfilariae immediately after blood-feeding. Fat body synthesis of vitellogenin was significantly reduced as early as 8 h post-infection in comparison with sham-injected flies. When total haemolymph protein from infected and sham injected flies was compared, titres were significantly depressed 6 h post-infection. However, later in the infection, titres were elevated by 30%, the major component being vitellogenin. The injection of dead microfilariae had no effect. An infection burden of a single parasite caused a significant reduction in ovarian protein content in comparison with shams, but no further significant decrease was observed as the parasite burden was increased from 5 to 20. Possible mechanisms underlying the disturbance of Simulium reproductive physiology are proposed.