We report the isolation of cDNA clones for fibronectin from a rat liver library prepared in the expression vector, Xgtll. Restriction mapping and DNA sequencing of these clones establish the sequence of the C-terminal 35% of rat fibronectin, covering the cell-, heparin-, and fibrin-binding domains. The celland heparin-binding regions have homologous repeating sequences. Based on the sequence data and Sl nuclease mapping, we conclude that there are at least three different fibronectin mRNAs in rat liver which differ in coding potential. The three RNAs appear to arise by alternative splicing within the coding region and are probably all encoded by a single gene. The implications of these results for the structure and function of fibronectin and the differences between various types of fibronectin are discussed.