The impact of moderate infection with Wuchereria bancrofti (Cobbold) (25 mf/50 microliters blood) on the biological activities of Culex pipiens L. were investigated. Data revealed that W. bancrofti parasites did not interfere significantly with the oviposition rate (81.97%, 80.228% in non-infected and infected mosquitoes, respectively), fecundity (73.56 +/- 4.56 and 74.23 +/- 5.05 eggs/female) duration of the gonotrophic cycle (6.34 +/- 0.29 and 6.05 +/- 0.26 days), or egg hatchability (88.06% and 90.07%) of infected mosquitoes. Furthermore adult survival to the extrinsic incubation period of infected females (91.8%), did not vary from that of non-infected ones (78.87%). It is suggested that the lack of variations is due to the relatively low level of microfilariae (mf) to which mosquitoes are generally exposed, as revealed by parasite burden. It is concluded that in Egypt the relationship of W. bancrofti to Cx. pipiens is that of a commensal rather than a parasite.