A life table ofBessa harveyi(Townsend), a parasite of yellowheaded spruce sawfly,Pikonema alaskensis(Rohwer), was constructed from field samples of eggs, firstinstar maggots, and adults. Major mortality factors of the parasite, in order of importance, were overwintering maggot mortality, superparasitism, egg infertility and sloughing, and fall emergence. Overwintering mortality included flooding, desiccation, freezing, predation, encapsulation by the host, and failure of adult flies to exit host cocoons. No obligatory diapause was detected whenB. harveyimaggots were reared in the host at room temperature (21–22°C) and under constant light. The mean time for maggot development was 8.4 days (SE = 0.33). The sex percentage of 151 laboratory-reared puparia was 51.7% females. TotalB. harveyiparasitism onP. alaskensislarvae was 16.7% in 1978 and 21.2% in 1979.B. harveyiis not well adapted toP. alaskensis. Therefore,B. harveyiis a polyphagous parasite that concentrates on the most predominant host in its range. We conclude that no practical method of improving the effectiveness ofB. harveyionP. alaskensiscurrently exists.