Rodent and avian malaria parasites have been reported to have an adverse affect upon the reproductive fitness of mosquitoes. In order to determine whether fecundity reduction occurs in Anopheles gambiae s. l. infected with human malaria a study of wild-caught mosquitoes was undertaken in the Muheza district of north east Tanzania. Fully engorged, indoor resting females were collected daily for 4 months and maintained for 5 days. A sporozoite rate of 11·5% was detected for the whole collection and of those females alive on day 6 an additional 17·5% were infected with oocysts alone. Oocyst, but not sporozoite, infection resulted in a 17·5% reduction in egg production. Fecundity reduction was not caused by a reduction in bloodmeal size in infected females and no size difference was detected between oocyst-infected and uninfected females although sporozoite-positive females were significantly larger. Comparisons in parity between uninfected and infected groups indicate that infection does not affect survival beyond the first gonotrophic cycle as no changes in survivorship occurred as a result of sporozoite infection.