Gyrodactylus salaris, an important pathogen of Atlantic salmon Salmo salar, has been shown to be highly sensitive to factors in host serum and mucus, being killed rapidly (50% within 1h) by serum at a dilution of 1[ratio ]200. The time needed for killing was inversely proportional to serum concentration. Similar effects were noted using host mucus, which contained approximately 1/20th of the anti-Gyrodactylus activity of serum. Serum activity was abolished completely by heating at 45°C for 30 min, and by addition of EDTA, but not by EGTA+1 mm magnesium ions. Activity was not dependent on whether the serum was from infected or naive fishes, nor was it species specific. Attempts to pre-coat parasites in salmon anti-Gyrodactylus antibodies also failed to enhance the activity of fresh serum. These observations suggest that killing is due to the complement system of the host, acting via the alternate pathway. G. salaris appears to be exceptionally sensitive to complement, being killed at concentrations which could be experienced in vivo. The role of complement in the protection of fishes against gyrodactylid infection therefore deserves further investigation.