Mullerian inhibiting substance (MIS), also known as anti-Mullerian hormone, is a glycoprotein belonging to transforming growth factor beta superfamily. In mammals, MIS is responsible for regression of Mullerian ducts, anlagen of the female reproductive ducts, in the male fetus. However, the role of MIS in gonadal sex differentiation of teleost fishes, which do not have the Mullerian ducts, has yet to be clarified. To address the role of MIS on gonadal sex differentiation in fishes, we isolated a MIS cDNA from the Japanese flounder testis and examined the expression pattern of MIS mRNA in gonads of both sexes during sex differentiation period. In this study, we present the first demonstration of sexually dimorphic expression of MIS mRNA during sex differentiation in teleost fishes, similarly to amniote vertebrates which possess the Mullerian ducts. (C) 2004 Elsevier Inc. All rights reserved.