Abstract.The effects ofOnchocercainfection onSimulium ornatumMeigen vitellogenesis were investigated. Vitellogenesis is triggered by ingestion of blood and is therefore linked to disease transmission. Vitellin subunits were described for the first time in the Simuliidae. In the British blackfiyS.ornatum, vitellin in the ovaries consists of two subunits, with a large subunit of molecular weight 200 kDa and a small subunit of 68 kDa.Simulium ornatumvitellin was recognized by polyclonal antibodies produced fromAedes aegypti(L.) vitellin. When blackflies were injected withOnchocerca lienalis(Stiles) microfilariae, there was a significant reduction in the amount of vitellin in the ovaries at 24, 34 and 50 h post blood‐feedin