A severe coagulopathy occurred in adult rainbow trout, experimetnally injected with Yersinia ruckeri via either the intraperitoneal or intracardial route, within 48 hours following injection. This coagulopathy was characterized by a marked lengthening of the prothrombin time, the partial ahromboplastin time, and the recalcification time with an accompanying extreme hypofibrinogenemia.  In preliminary experiments, in which endotoxin prepared from Y. ruckeri by the Westphal hot phenol method was injected intracardially in rainbow trout, similar changes in plasma coagulation have been observed. These results indicate that coagulation disorders observed in fish during experimental infections of Y. ruckeri may be in part analogous to the endotoxin induced Disseminated Intravascular Coagulation syndrome which often occurs in mammals during gram negative septicemias.