We have isolated cDNA and genomic clones for rat fibronectins. A single gene gives rise to three different mRNA species by alternative splicing at a complex intron-exon boundary within the coding region. The fibronectins encoded by these three mRNAs differ by the insertion of different protein segments in the C-terminal heparin-binding domain. The amino acid sequences of the cell-, heparin- and fibrin-binding domains in the C-terminal third of fibronectin were deduced from the DNA sequences. The cell- and heparin-binding regions each consist of several similar repeating sequences known as type III homologies, while the fibrin-binding region comprises three repeats of a different type (type I homologies). The sequences suggest several hypotheses for the structure-function relationships of these domains of fibronectin; these hypotheses are now being tested. Determination of intron-exon boundaries within the fibronectin gene is beginning to reveal the modular structure of the gene and its relation to the repeating structure of fibronectin.