--We documented blackfly infestations (Simulium canonicolum) at 42 Red-tailed Hawk (Buteo jamaicensis) nests in Wyoming. Blackflies caused mortality at 6 of 42 (14%) nests where young hatched (13 of 87 nestlings) and were the only known cause of nestling mortality. The onset of infestations occurred when nestlings were 3 to 20 days old and usually lasted until nestlings died or fledged. Age of nestlings at mortality ranged from 9 to 43 days. Levels of blackfly infestation were highly variable among nests and were affected by weather The cumulative effects of infestations on nestlings, including physical harassment, Leucocytozoon (a blood protozoan transmitted by blackflies) infection, and direct loss of blood and body fluids from biting flies, apparently resulted in mortalities through sustained physiological damage, trauma associated with early nest departure, or both. Because blackfly infestations can be detected only at close range, are ephemeral at nests, and can cause mortality of nestlings over a wide range of ages, the presence of blackflies and their influence on reproduction probably are undetected during most raptor productivity surveys. Received 23 January 1997, accepted 26 August 1997. A VARIETY OF PARASITES has been found in raptors (Trainer 1969, Keymer 1972, Greenwood 1977, Guti6rrez 1989, Philips 1990, Forrester et al. 1994). Although the literature describing parasites in raptors is fairly extensive, few researchers have described the effects of these parasites on their raptor hosts. Blackflies (Simulium spp.) and the parasites they transmit have been associated with at least four species of raptors (Greiner and Kocan 1977, Jolly 1982, Hunter et al. 1997), but their effect on the health of these species is largely unknown. Fitch et al. (1946) reported that 7 of 11 mortalities of nestling Red-tailed Hawk (Buteo jamaicensis) were due to blackflies (Eusimulium clarum). Similarly, Brown and Amadon (1968) reported that during wet years, biting flies (Prosimulium spp.) were a principal cause of mortality of nestling Red-tailed Hawks in Cal4 Present address: Teton Science School, P.O. Box 68, Kelly, Wyoming 83011, USA. E-mail: roger@tetonscience. org s Present address: Department of Wildlife, Humboldt State University, Arcata, California 95521, USA. ifornia. However, they did not suggest possible mechanisms of mortality and gave no source for the information. Stabler and Holt (1965) reported a prevalence of blood protozoa in a number of different hawks, and Forrester et al. (1994) documented the relative occurrence of blood parasites in raptors in Florida but made no mention of the potential effects of these parasites on raptor productivity. Hunter et al. (1997) attributed mortality of Great Horned Owls (Bubo virginianus) to feeding blackflies (Simulidae), anemia, and blood parasite (Leucocytozoon spp.) infection. However, all of the mortality they observed occurred in owls tethered to platforms for experimental purposes. In a portion of our study area in Grand Teton National Park (GTNP), northwestern Wyoming, the number of young Red-tailed Hawks fledged per nest declined from 1.4 to 0.7 between 1947 and 1975 (Craighead and Mindell 1981). More recent monitoring of productivity (1990 to 1991) indicated a high proportion of nesting failures, and dead young were found in and beneath some nests (S. Cain unpubl. data). In all cases where dead nestlings were found, prey items were in the nests, and adults were