A retrospective study of 28 cases of rattlesnake bites in dogs indicated an association of envenomation with echinocytosis; 25/28 dogs (89%) had echinocytosis within 24 hours of a rattlesnake bite being reported. Eighteen of the 28 dogs had marked type III echinocytosis (95-100% of mature erythrocytes affected), and seven dogs had moderate echinocytosis (15-30% of mature erythrocytes affected). The echinocytosis was transient, resolving within 48 hours of envenomation in those for which subsequent blood films were available (13/25). Hypokalemia was noted in 14 of 18 dogs for which biochemical data were available. The hypokalemia was not consistently associated with echinocytosis. The observation of echinocytosis in dogs supports a clinical diagnosis of rattlesnake envenomation.