Miniparamyosin (mPM) is a protein of invertebrate muscle thick filaments. Its similarity to paramyosin (PM) suggests that it regulates thick filament and myofibril assembly. To determine its role in muscle structure and function we overexpressed mPM in muscles of Drosophila melanogaster. Surprisingly, myofibrils accumulating excess mPM assemble nearly normally, with thick filament electron density and sarcomere length unaffected. Myofibrils in some indirect flight muscle groups are misaligned and young flies exhibit a moderate level of flight impairment. This phenotype is exacerbated with age. Transgenic flies undergo progressive myofibril deterioration that increases flight muscle dysfunction. Our observations indicate that the correct stoichiometry of mPM is important for maintenance of myofibril integrity and for the proper function of the flight musculature.