The muscle band surrounding the swimbladder of the toadfish (Opsanus tau) is one of the fastest known muscles in vertebrates. Rapid, non-fused twitches are responsible for the characteristic sound produced by the organ by both male and female toadfish. We have quantitated the membrane systems (transverse (T) tubules, sarcoplasmic reticulum (SR) and mitochondria), and some of their proteins (Ca2+ ATPase, or calcium pump, and foot protein or Ca2+ release channel) in these muscle fibres. As expected from the well-known morphology, the content of Ca2+ release and Ca2+ uptake proteins are considerably higher than in slower twitch fibres (e.g. fast-twitch and slow-twitch fibres in hind legs of mammals). Unexpectedly, the increment in ATPase is much larger than the increment in foot protein. The ATPase to foot ratio in muscle fibres from the swimbladder of males and females is higher by a factor of five to seven than in guinea pig fast-twitch fibres. We conclude that calcium uptake is a limiting factor in the ability to sustain the trains of high frequency, non-fused synchronous contractions of which these fibres are capable. Sexual dimorphism is demonstrated in the content of mitochondria (higher in males) and in the density of junctional feet (higher in females). The former is probably related to the more continuous activity during the males' mating call but the latter is to be demonstrated.