A single case of adenocarcinoma of the retinal pigment epithelium occurred in a guppy, Poecilia reticulata, Peters. This is the first such tumour reported from fishes. The left eye of the affected fish was severely exophthalmic because of a large intraocular tumour mass. The tumour, which displaced normal retina anteriorly, consisted mainly of melanin-containing epithelial cells. Neoplastic cells were bilayered and arranged in a tubular pattern. The tumour was confined to the orbit. Although the specimen was from a group exposed to a mixture of halogenated organic compounds, the lesion was not considered to have been chemically induced because of its rare occurrence within the group as a whole. (Copyright (c) 1992 Academic Press Limited.)