- Ecological and production studies of Prosimulium caudatum Shewell, P. dicum Dyar and Shannon, Simulium arcticum Malloch and S. canadense Hearle (Diptera: Simuliidae) were made in 1971 and 1972 in three streams near Corvallis, Oregon. The purpose of the study was to characterize the biology and population dynamics of larvae, and to compare the production of the four species under various spatial and temporal conditions. A total of 54 annual production estimates was derived utilizing 37 stations. In addition, 17 annual drift and 22 emergence (as pupae) determinations were made. Data on the physical and chemical characteristics of the streams, and on larval associates, parasites, predators and food types were obtained at all stations. Laboratory experiments provided data on feeding rates and predator impact. The two Prosimulium species always coexisted, and occurred in all streams. Simulium arcticum and S. canadense were studied in two streams only. All species were univoltine and had egg stage durations exceeding 40 weeks. Prosimulium spp. larvae appeared in mid-February and were on riffles for an average of 10.3 weeks. Simulium arcticum hatching began about six weeks later. Larvae of this species developed in an average of 8.8 weeks. Development of S. canadense began about nine weeks later than for Prosimulium spp. but lasted nearly as long. All four black flies had six larval instars. All larvae were filter feeders, and fed primarily on the suspended materials in the streams. Detritus made up from 65-90% of the diet, with the rest being filamentous algae. Alimentary tract clearing of larvae was usually accomplished in less than one hour. Black fly immatures were associated with about 75 other benthic insect species. Of these, nine were likely important simuliid natural enemies. Fish were minor black fly predators. Gastromermis nematodes occurred in about 3% of all mature simuliid larvae. Annual losses due to drift averaged about 6%. Simuliid drift appeared to be a function of standing crop density. Growth-related crowding may also have been a factor. Effects of diel light conditions, temperature and discharge were minimal. A diel emergence pattern existed for Simulium spp. adults only. The time between emergence and oviposition was likely about 0.5 week for all species. Emergence at most stations was single peaked, and averaged 3. 6 weeks in duration. The sampling universe at each station could be effectively partitioned to exclude areas where larvae did not exist. Thus, the weekly stratified random sampling program provided population estimates with error terms of usually 15% or less. All quantitative data were expressed on a per m² of overall riffle basis (and not on a per m² of habitat basis) to make the study more comparable to other benthic research. Density, biomass and production differences between stations, streams and years were minimal for each species, and therefore were treated as replicates. Initial densities ranged from over 11000 to about 16000 larvae/m². Mortality for aggregates of all species was similar, with about 70% of all losses occurring during the first half of larval life. Growth for all species was generally logistic and averaged over 0.090 gig per day annually. Production, averaged for 1971 and 1972, in g/m² , was: P. caudatum, 2.644; P. dicum, 2.908; S. canadense, 3. 141; and S. arcticum, 3.950. This species ranking also held for initial density, mean density and mean biomass. Biomass and production were almost always closely correlated. Mortality variations had a greater influence on production than did growth rate. Averaged for all species, the mean total mortality due to drift and parasites was 6.2% and 0.5% respectively. An average of 10.7% of colonizing larvae survived to emerge. Predator-caused losses were estimated to provide the 82.6% needed to balance the loss budget. Black fly larvae were the dominant processor of suspended materials in the three streams. On the average, they outnumbered all other primary consumers combined on riffles by a factor of 27. Simuliid larvae functioned as the primary prey reservoir for as many as 20 species of insect carnivores from February through June of both years.