One mechanism used by teleost fishes to produce acoustic communication signals involves the contraction of sonic "drum" muscles that appose the lateral walls of the swimbladder. In one marine species, the midshipman (Porichthys notatus), there is a sex difference in the overall size of the swimbladder as well as in the ultrastructural properties of its myofibrils. Additionally, there are two classes of sexually mature males referred to as Type I and Type II. The peripheral sonic motor system of Type I males differs from that of Type II males and females (which resemble each other) in a number of ways: (1) the mass of their swimbladder and associated sonic muscles is 50% greater, (2) their muscle fibers are several times larger and have a characteristically large volume of sarcoplasm that surrounds the myofibrils and is densely filled with mitochondria, (3) the length of z-lines of their myofibrils is about 20-fold greater, and (4) their sarcoplasmic reticulum is more highly branched. The ultrastructure of the myofibrils of Type II males and females resembles that found in the sonic muscle of males and females in other related species. The larger mass and specializations of the sonic muscle in Type I males are considered to be adaptations related to their known role in sound production and the unique long duration "humming" sounds that they generate during the breeding season. The similarity in the sonic motor system between females and Type II males is considered to be related to the utilization of an "alternative mating strategy" by Type II males. To our knowledge, this is the first documentation of a sex difference or, for that matter, a sexual polymorphism in the ultrastructural features of a vertebrate myofibril.