Abstract We have cloned by the polymerase chain reaction a 2.1-kb fragment carrying heavy-chain joining ( J H) gene segments and a part of the J H- C μ intron of the rat. Sequencing showed that the rat genome has four functional J H segments and that only a slight divergence has occurred after the separation of rat and mouse. A systematic sequence comparison between the two species and human revealed an additional J H pseudogene in rat and mouse 5′ of J H1 which has not been described so far. The implications in evolutionary terms are discussed. In addition, we give an assessment of the misincorporation rate of the Taq polymerase.