A detailed cytoarchitectonic description of the diencephalon in the channel catfish reveals more than 40 distinct cell groups. Of these, 4 are located in the preoptic area, 8 in the hypothalamus, 7 in the thalamus, 2 in the epithalamus, 15 in the posterior tuberculum and 7 in the synencephalon. A comparison of the diencephalon of Ictalurus punctatus to that of goldfish, which has previously been described, indicates major differences in the diencephalon between these two species. Channel catfish lack the superficial pretectum and the nucleus 'glomerulosus' of goldfish, whereas the anterior tuberal nucleus of the hypothalamus, the paracommissural nucleus of the synencephalon and the nucleus lobobulbaris of the posterior tuberculum are all better developed in channel catfish than in goldfish. The nucleus electrosensorius in the synencephalon of channel catfish is probably homologous to the nucleus of the same name in gymnotoid teleosts, but it appears to have no homologue in the diencephalon of goldfish. An analysis of interspecific variation in the diencephalon among catfishes, goldfish and several other teleosts reveals that some areas of the diencephalon are more variable across species than others. Specifically, the migrated portions of both the posterior tuberculum and the synencephalon appear to be most variable, whereas the epithalamus and the preoptic area are the most conservative. It is hypothesized that the relatively conservative areas of the brain may have a greater number of afferent connections, and probably also a greater number of distinct behavioral functions, than the more variable areas.