The dorsal muscle of five South Australian bottle-nose dolphins has been studied. The ultrastructure and cytochemistry of the red, white and intermediate fibres were essentially similar to those of other mammalian skeletal muscles. Two features of special interest were: (a) the large number of lipid droplets in the red muscle fibres, and (b) the presence of longitudinal tubes linking the transverse tubes of the T-system, both within a sarcomere, and also between adjacent sarcomeres, of the white muscle fibres. The muscle spindles and neuromuscular junctions in the dolphin appeared to be morphologically similar to those of other mammals.