We have used Anopheles gambiae, a major vector of malaria in Africa, to test the hypothesis that the operation of a surveillance or immune system against microorganisms and parasites can be costly to the reproductive success of the host. Blood-fed mosquitoes were challenged with an immune elicitor, lipopolysaccharide (LPS), and their resultant antimicrobial activity, accumulation of yolk protein in the ovary and egg production was monitored. Humoral activity against the Gram-positive bacterium Micrococcus luteus was induced by LPS injection in a dose-responsive manner. LPS treatment also caused a concomitant significant reduction in the accumulation of protein in ovaries 24 h after injection and in the production of eggs during the same gonotrophic cycle. Unlike immune stimulation, reduction in reproductive fitness was not dose responsive. Oral administration of LPS also significantly reduced ovarian protein content although we could not detect the presence of anti-M. luteus activity in the gut tissue by using an inhibition zone assay. These findings indicate that immune stimulation imposed reproductive fitness costs on mosquitoes.