The zebrafish Danio rerio is an important model system for genetic and developmental studies of the vertebrate central nervous system. Considerable knowledge concerning the embryonic development of the central nervous system of the zebrafish has accumulated in recent years. However, there is an apparent lack of information on the organization of the adult zebrafish brain. We have therefore recently studied in detail the neuroanatomy of the adult zebrafish. Here we compare the brains of the zebrafish and of the closely related and neurobiologically well-investigated goldfish, Carassius auratus. Two sensory systems, the visual and the gustatory systems, were identified as differing on the gross morphological and histological levels in the two species. The goldfish shows the simple (evolutionarily reduced) pattern of pretectal organization, and its gustatory system is massively enlarged. The pretectum of the zebrafish conforms to this simplified visual pretectal pattern, although the retention of some ancestral pretectal characters indicates a lesser degree of reduction of the visual system compared to the goldfish. The gustatory system shows many similarities with the evolutionarily derived and functionally specialized gustatory system of the goldfish. However, some peripheral and central gustatory characters are missing in the zebrafish, indicating a less specialized gustatory system.