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. Eosinophilic foci consist of hypertrophied cells, enlarged atypical nuclei, and dense glycogen-free cytoplasm. Mitotic figures are also occasionally seen. Usually, these foci have been infiltrated and at least partially destroyed by inflammatory cells, largely lymphocytes. In some liver sections, eosinophilic foci are intact and occasionally an eosinophilic-basophilic transformation can be seen. However, most often basophilic foci appear independently, surrounded by normal hepatocytes, with no indication of a prior eosinophilic stage. The cells of basophilic foci are similar to those of carcinomas: intensely basophilic, mitotically active, devoid of glycogen, and grouped into cords several cells in thickness. These nodules may appropriately be referred to as carcinomas in situ, because the only distinguishing characteristic is the size of the lesion. Attempts at differentiation between benign and malignant liver lesions appear arbitrary. We believe the best classification of the neoplastic liver lesion in trout is a hepatocellular carcinoma because the potential for malignant behavior always exists and, with sufficient time, can often be histologically demonstrated. We have also described our experience with the characteristics of other liver lesions associated with hepatocarcinogenesis.