Musca domesticaL. were exposed to intense selective pressure (70–90%) from a wasp parasite (Nasonia vitripennisWalker). By the 40th generation parasite progeny produced on the resistant host race was only 57 compared with 231 progeny on the control (unexposed) hosts—a 75% reduction in progeny production. Longevity of female parasites on the resistant hosts was only about 5 days, whereas on control hosts longevity was about 9 days. Knowledge of co-evolution in parasite-host and predator-prey systems can assist entomologists in selecting effective natural enemies for biological control.