The serotonergic system in the telencephalon of the domestic chick was investigated using an antibody specific to serotonin (5-HT). Most parts of the forebrain, such as the different subdivisions of the visual Wulst and the neostriatum, displayed a rather uniform, moderate to dense innervation of serotonergic (5-HT+) fibers. However, some highly distinct area-specific differences could be observed. Primary sensory areas such as the ectostriatum, layer L2 of field L, and the rostral part of the nucleus basalis displayed very few 5-HT+ fibers. In contrast, the dorsal part of the archistriatum intermedium, the nucleus taeniae, a medial part of the lobus parolfactorius and the dorsomedial part of the hippocampus displayed an extremely dense serotonergic innervation. In general, three different types of 5-HT+ axons could be distinguished. The most common was a fine, highly varicose type, whereas beaded axons, exhibiting larger varicosities, and a thick non-varicose type, exhibiting occasional swellings, were much sparser. In summary, these findings indicate that the serotonergic innervation of the avian telencephalon is extensive but site-specific, and is organized in a highly similar way to that in mammals. The high accumulation of 5-HT+ fibers in the dorsal part of the archistriatum intermedium points to a prominent role for 5-HT in fear behavior.