Hydroelectric generating stations along the Winnipeg River are subject to emergence of large numbers of caddisflies which cause work-related allergies. The purpose of this study was to determine peaks in seasonal and nightly flight activity of the most abundant caddisfly species in and around the generating stations and to recommend management practices to alleviate the problems caused by caddisflies. Modified New Jersey light traps were used to capture caddisflies during the 1997 and 1998 field seasons at hydroelectric generating stations at Great Falls and Seven Sisters. The estimated total number of caddisflies caught over the two-year period was 526,607; 275,806 in 1997 and 250,801 in 1998. The caddisflies belonged to 14 families, 35 genera and at least 76 taxa. The caddisfly flight season, from first capture to last, differed by only one week in the two years. In 1997, caddisflies were collected from 1 June to 16 October, and in 1998 from 26 May to 8 October. Peak flight activity occurred one week earlier at Great Falls than at Seven Sisters. In 1997, approximately 75% of caddisflies were captured during the five-week period from the last week of June to the end of July. The peak flight activity in 1998 occurred from mid-June to mid-July (four weeks), when approximately 80% of the yearly total of caddisflies were captured. Peaks in nightly flight activity were seen between 2300 and 0100, when approximately 62% of the nightly total were captured. Large numbers of caddisflies were captured inside the generating stations. To determine the caddisfly mode of entry, four nights were spent in the stations. Through personal observations and the use of emergence traps, it was established that caddisflies enter the buildings through various openings (i.e. broken windows, under doors) and by emergence from the gate openings inside the gate rooms. Insect particulates, including identifiable caddisfly parts, which were blown into the generating stations through the air-cooling system, were also collected in fine nylon filters attached to the turbine caps in the powerhouses. Changes in management practices are required to decrease the exposure of Manitoba Hydro employees to caddisfly particulate. These must include maintaining and improving sanitation practices and increased vigilance to remove or exclude caddisflies from the generating stations.