Immunocytochemical techniques were used to investigate the distribution of calretinin (CR) in the telencephalon of adult and developing brown trout (Salmo trutta fario L.). Previous immunoblotting analysis of trout brain extracts with a CR antibody revealed a single protein band of 29 kDa, similar to that observed in rat brain extracts. In the forebrain of adult trout, CR immunoreactivity was distributed in well-defined cell groups, which allowed us to analyze the CR-immunoreactive (ir) neuronal populations in terms of their respective regions of origin. Our results show that the CR-ir populations of the dorsal and ventral telencephalon are differentially distributed along the rostrocaudal axis, indicating the existence of four main populations of pallial origin and several ventral (subpallial) populations. A highly specific pattern of innervation by CR-ir fibers of different telencephalic regions was observed from alevins to adults. The first CR-ir cell groups of the telencephalic hemispheres were observed in the ventral telencephalic area and preoptic region of 7–8-mm embryos. In later embryos and in alevins, further CR-ir cell groups appeared in the ventral and dorsal telencephalic areas, showing a dorsoventrally banded pattern at precommissural levels. Study of CR expression provided new criteria for understanding the organization of the telencephalon of trout, and hence of teleosts. J. Comp. Neurol. 467:254–269, 2003. © 2003 Wiley-Liss, Inc.