Abstract Evidence is presented, confirming the presence of an anticoagulant system in the plasma of Atlantic salmon ( Salmo salar L) and rainbow trout ( Oncorhynchus mykiss Walbaum) (Order: Teleostei, Family: Salmonidae) that bears striking similarities with the protein C anticoagulant system in mammals; its vitamin K-dependence was documented through a warfarin feeding trial. A potent activator of this system is the protein C activator from the venom of the Central American Moccasin, Agkistrodon bilineatus. When activated, the system splits the tripeptide substrate glu-pro-arg-pNa, which is the substrate preferred for the in vitro assay of human protein C. It also prolongates the plasma activated partial thromboplastin time, indicating that the system is of clinical relevance. A temperature dependence of the plasma protein C-like activity was evident, the mean activity being 5- to 10-fold higher, but also more variable, in both species in summer and early fall, than it was in winter. There was also a species difference, with rainbow trout having the higher levels. In man, subnormal values of protein C implies an increased risk of thrombosis. Whether this applies to fish is not known. It is, however, a fact that microvascular thromboses are prevalent in farmed Atlantic salmon in winter, whereas thrombotic disease is not a problem in rainbow trout; in the present study plasma protein C-like activity was 30% (of a human reference plasma) in salmon at 4°C compared to a level of 60% in rainbow trout. A complicating factor for the assay of protein C-like activity in salmonid plasma, is the poor stability of the inhibitory system upon storage. Consequently, assays have to be done with freshly prepared citrated plasma.