Claustrum is a telencephalic structure integrating information of various modalities. Proper functioning of this structure depends on the presence of a network of intrinsic connections. This includes GABA-ergic neuronal populations that also contain calcium-binding proteins (CaBPs). The goal of this study was to analyze qualitative and quantitative the 5-HT-containing fibers in the rat claustrum and to assess the relationships between these fibers and the populations of claustral neurons expressing CaBPs. We used the methods of immunocytochemistry and morphometry. The serotonergic fibers in the claustrum are heterogeneous, both with respect to their morphology and spatial distribution. Thin varicose fibers are more numerous and are homogeneously distributed within the claustrum. Remaining fibers were thicker and possessed larger varicosities. They were present mainly in the ventral part of the claustrum. Although the serotonergic fibers are found in the vicinity of claustral cells containing CaBPs, direct contacts between these fibers and cells are rare. Other mechanisms, including volume transmission, may possibly mediate serotonergic influences.