Blooms of Cochlodinium sp., monitored for the first time on the west coast of Vancouver Island from August to October 1999, caused substantial mortality to farmed salmon accounting for economic losses of about $2 million. Cells of the alga were 30-45 µm in length, 20-30 µm in width and had a torsion of 1.8 to 2 turns. Double cells up to 25% of the biomass were common, and apart from the absence of longer chains the anteriorly placed nucleus and numerous golden chloroplasts matched the morphological description of Cochlodinium polykrikoides Margalef. A strong diurnal pattern was observed in blooms at farm sites with high cell concentrations overnight at depths to 25 m and thick surface rafts during the day. Surface concentrations of 60,000 cells ml-1 peaked in early September. Fish stopped feeding when cell counts exceeded 500 cells ml-1 in the netpens, and mortality was observed above 2000 cells ml-1. Bioassays in the field with Atlantic salmon, Salmo salar, smolts demonstrated lethality after 120 min exposure and over 90 % mortality after 500 min when cell concentrations varied from 10,800 to 2,700 cells ml-1 as the bloom moved through the test site. Under controlled laboratory conditions Salmo salar smolts died within 27 min with exposure to 7,200 cells ml-1, 55 min with 3,400 cells ml-1, and although fish appeared distressed in 1,000 cells ml-1 only 20% died within the 24 h bioassay. Mitigation protocols of netpen enclosures with 15 m deep tarps and upwelling of deep water by aeration proved less effective against Cochlodinium than for Heterosigma carterae, the major killer of farmed salmon on the west coast of Vancouver Island.