The histological progression of hepatic neoplasia has not been as systematically studied in rainbow trout as it has been in rodents. Two putative preneoplastic lesions have been identified, the eosinophilic focus and the basophilic focus, but whether these correspond to similar lesions in rodent livers is not known. Preneoplastic liver lesions in rodents have been extensively characterized histochemically, but adaptation of these techniques to trout livers has not always been successful. Attempts at differentiation between benign and malignant liver lesions appear arbitrary. The authors believe the best classification of the neoplastic liver lesion is trout in a hepatocellular carcinoma because the potential for malignant behavior always exists and, with sufficient time, can often be histologically demonstrated. The authors have also described our experience with the characteristics of other liver lesions associated with hepatocarcinogenesis.