Fish, a potential Src substrate, is a broadly expressed adaptor protein containing five SH3 domains and a phox homology (PX) domain (1). The Src family of protein tyrosine kinases act in signal transduction pathways (2-4). Src kinases vary in expression but are strongly regulated in vivo; catalytic activity is repressed by interacting with the SH3 domain (5-7). In Src-transformed fibroblasts and in normal cells treated with certain growth factors fish is tyrosine-phosphorylated (1). Treatment of cells with cytochalasin D results in rapid tyrosine phosphorylation of fish, along with activation of Src (1). Fish is likely to be involved in tyrosine kinase signaling and may have a role in cytoskeletal changes (1).