Trophoblasts, a cell type found in the placenta, play a pivotal role in the function of the placenta as a barrier between the maternal fluid and the fetus. Recently, claudin, a 24-kDa transmembrane protein, was identified as being responsible for the barrier function of epithelia. In the present study, we investigated the expression profiles of claudin and the changes in expression during the differentiation of BeWo human trophoblast cells. Reverse transcriptase-polymerase chain reaction and immunoblotting demonstrated the expression of claudin-1, -3, -4, and -5 in BeWo cells. Forskolin, which induces the differentiation of BeWo cells from cytotrophoblast-like cells into syncytiotrophoblast-like cells, reduced slightly the expression of claudin-5. This is the first report to show changes in claudin-5 in forskolin-treated BeWo cells.