Fluorescent antibody staining experiments with both isolated myofibrils and muscle fibers grown in culture show that AMP deaminase is bound to the myofibril in the A band. The strongest staining occurs at each end of the A band. The approximate width of the fluorescent stripes and their relation to the A band remains constant as a function of sarcomere length. Removal of enzyme from the myofibrils leads to loss of staining, and readdition of purified enzyme restores the original staining pattern. A histoenzymatic method for the detection of AMP deaminase activity in cultured fibers gives comparable localization. The results are consistent with the previous observation (Ashby, B. and C. Frieden, 1977. J. Biol. Chem. 252: 1869-1872) that AMP deaminase forms a tight complex in solution with subfragment-2 (S-2) of myosin or with heavy meromyosin (HMM).