Smad proteins are the cellular mediators of the transforming growth fac-tor-b superfamily signals. Herein, we describe the isolation of a fourth Smad gene from the helminthSchistosoma mansoni, a receptor-regulated Smad (R-Smad) gene termedSmSmad1B. The SmSmad1B protein is com-posed of 380 amino acids, and contains conserved MH1 and MH2 domains separated by a short 42 amino acid linker region.