Tropomodulin (Tmod) is an actin pointed end capping protein that regulates actin dynamics at thin filament pointed ends in striated muscle. We used the Drosophila Tmod homologue, sanpodo (spdo), to investigate Tmod's function during muscle development in the indirect flight muscle (IFM). Immunofluorescence of stained myofibrils and western analysis showed that SPDO was associated with the pointed ends of elongating thin filaments throughout myofibril assembly. Transient overexpression of SPDO during myofibril assembly irreversibly arrested elongation of pre-existing thin filaments whereas the lengths of thin filaments assembled after SPDO levels had declined were normal. Further, flies with a preponderance of abnormally short thin filaments were unable to fly. Our results indicate that thin filaments elongate from their pointed ends during myofibril assembly and that thin filament elongation and final length specification depends on developmental regulation of pointed end capping by SPDO. A transient increase in SPDO levels during myofibril assembly appears to convert SPDO from a dynamic cap to a permanent cap. We are currently screening existing muscle mutant lines to determine whether other proteins are required for the developmental regulation of pointed end capping during myofibril assembly.