Alternative splicing is one of the central mechanisms that regulate eukaryotic gene expression. Here we report a tissue‐specific RNA‐binding protein, Fox‐1, which regulates alternative splicing in vertebrates. Fox‐1 bound specifically to a pentanucleotide GCAUG in vitro. In zebrafish and mouse, fox‐1 is expressed in heart and skeletal muscles. As candidates for muscle‐specific targets of Fox‐1, we considered two genes, the human mitochondrial ATP synthase γ‐subunit gene (F1γ) and the rat α‐actinin gene, because their primary transcripts contain several copies of GCAUG. In transfection experiments, Fox‐1 induced muscle‐specific exon skipping of the F1γ gene via binding to GCAUG sequences upstream of the regulated exon. Fox‐1 also regulated mutually exclusive splicing of the α‐actinin gene, antagonizing the repressive effect of polypyrimidine tract‐binding protein (PTB). It has been reported that GCAUG is essential for the alternative splicing regulation of several genes including fibronectin. We found that Fox‐1 promoted inclusion of the fibronectin EIIIB exon. Thus, we conclude that Fox‐1 plays key roles in both positive and negative regulation of tissue‐specific splicing via GCAUG.