Control of actin filament length and dynamics is important for cell motility and architecture and is regulated in part by capping proteins that block elongation and depolymerization at both the fast-growing (barbed) and slow-growing (pointed) ends. Tropomodulin is a capping protein for the pointed end of the actin filament; it is associated with the free, pointed ends of the thin filaments in striated muscle, where it is thought to bind to both tropomyosin and actin. In embryonic chick cardiac myocytes, tropomodulin assembles after the thin, as well as the thick, filaments have become organized into periodic I and A bands, suggesting that tropomodulin might be involved in maintaining actin filament length. Here we show that microinjection of an antibody that inhibits tropomodulin's pointed-end-capping activity in vitro results in a marked elongation of actin filaments from their pointed ends and a > 80% reduction in the percentage of beating cells. This demonstrates that pointed-end capping by tropomodulin is required to maintain actin filament length in vivo and that this is essential for contractile function in embryonic chick cardiac myocytes.