Larvae of the introduced blow fly Chrysomya rufifacies reduced native blow fly Cochliomyia macellaria survivorship at a variety of densities and proportions at both 22 and 29°C. No effect of C. macellaria on C. rufifacies survivorship was observed. Addition of C. macellaria increased the time period from egg to adult in C. rufifacies. The reduction in C. macellaria survivorship was so great as to outweigh any drop in C. rufifacies fitness resulting from increased development time; the interaction appears to be a highly asymmetric case of competition. -from Authors