Spectrin comprises a family of polypeptides thought to be involved in mediating linkage of actin filaments to the plasma membrane in a wide variety of cell types (for reviews see refs 1-3). Spectrin is present as a tetramer composed of two non-identical subunits. Most cells express a common subunit with a molecular weight (Mr) of 240,000 (240 K; termed alpha-spectrin) in association with a polymorphic cell type-specific subunit: Mr 260 K in the intestinal terminal web (termed TW260), Mr 235 K in nervous tissue., liver, lymphocytes and fibroblasts (termed gamma-spectrin; also referred to as fodrin), and Mr 225/220 K in erythrocytes and adult cardiac and skeletal muscle (termed beta'- and beta-spectrin, respectively). We show here that primary chicken myoblasts express predominantly alpha gamma-spectrin, but on terminal differentiation in vitro the cells gradually switch to alpha beta-spectrin as a result of the onset of beta- and beta'-spectrin synthesis and by the subsequent differential stabilization of beta- and gamma-spectrin. This switching correlates with known changes in the biophysical properties and function of the developing muscle sarcolemma and cytoskeleton.