Laboratory observations on the effect of Hepatozoon gracilis on the egg production of the mosquito Cx. (Cx.) pipiens Linneaus under laboratory conditions revealed that H. gracilis infected mosquitoes produced significantly fewer eggs than uninfected ones. The egg production decreased as parasite burdens increased. Reduction in blood meal size in infected females did not reduce fecundity. No size differences was detected between oocyst-infected and uninfected females although sporozoite positive females were significantly large. Preoviposition period was affected significantly, while incubation period and percentage of egg hatching showed no significant changes. The longevity of female infected mosquitoes decreased insignificantly than in uninfected ones.