The mosquitoCulex pipiens quinquefasciatusSay was found to have developed resistance to the mermithid nematodeRomanomermis culicivoraxRoss and Smith after some 300 generations. Parasitism was 32–42% higher in a native strain than in the laboratory strain of the host under similar conditions. Differences were even greater when the number of successful penetrations (multiple parasitism) was measured. No evidence of humoral defense mechanisms were found, and the mode of resistance has not been determined.