A single nucleotide polymorphism in the transforming growth factor-$eta$ type II receptor (TGE$eta$RII) gene of the rat was studied. TGF$eta$RII is a tumor suppressor that is frequently inactivated by mutation in human colon cancers. A novel nucleotide polymorphism of G to A(or A to G), which causes a silent mutation at codon 129, was found in G:C rich sequence in the TGF$eta$RII gene of Sprague-Dawley rats. The results suggest that genetic polymorphism occures without a strain of the laboratory animal.