We describe a novel isoform of the beta 1 integrin subunit called beta 1D, which contains a unique cytoplasmic domain, and is expressed specifically in skeletal and cardiac muscle. The beta 1D isoform arises from splicing into the final transcript of an additional exon located between exons 6 and 7. The nucleotide sequence of beta 1D predicts a cytoplasmic domain of 50 amino acids in which the last 21 amino acids of the beta 1A integrin isoform are replaced by a related sequence of 24 amino acids. A beta 1D-specific anti-peptide polyclonal antibody was prepared and immunoprecipitation of tissue extracts with subsequent immunoblotting showed expression of beta 1D isoform only in striated muscle cells.