The anterior and the posterior drum muscles of the piranha resemble each other in all essential fine structural aspects: myofibrils are slender; sarcomeres are short compared with those of other drum muscles; mitochondria, located in the periphery of the fibers, are numerous and show an irregular internal structure; and the sarcoplasmic reticulum is abundant. Triads appear at the level of the Z lines. The drum muscles have many motor endplates, which, however, lack the characteristic junctional-fold apparatus. No lipid substances could be demonstrated in these muscles. In the posterior drum muscle the fibers depart from their orderly longitudinal arrangement at irregular intervals.