The following descriptions are based on material collected during a field trip to the east coast of Vancouver Island, British Columbia in June 1955. Most of the streams concerned are in the vicinity of Qualicum Bay, a summer resort midway between the cities of Nanaimo and Courtenay. Lying in the lee of the mountainous interior of the island, this narrow strip of coastal forest has a climate somewhat drier than is normal for the region as a whole. The annual precipitation is less than 50 inches and there is a pronounced dry period in July and August. The short coastal rivers are in spate in the fall and winter months when the heaviest rainfall occurs. They are fast and shallow with clean beds of gravel or small boulders. Because of limited water storage in the hinterland,flash-flooding is frequent after rain storms. Vegetation of the forest floor is lush and the courses of the smaller streams are often concealed by dense thickets of salmonberry (Rubus spectabilis Pursh.), thimbleberry (Rubus parviflorus Nutt.) and other shrubs that overhang the banks. During the spring and earlysummer, conditions appear to be very favourable for the propagation of black flies. In late May, one of the Prosimulium species described herein was already past its peak of emergence and the occurrence of even earlier species seems probable.