Three main types of fibre can be differentiated in the adult dogfish myotome at the immediate post-anal level. An outer band of muscle consists of 80-90 pale multiply innervated fibres (superficial fibres). These fibres are 80-90 micron in diameter, lack M-lines and have a low Ca2+-activated myosin ATPase activity. Volume densities of myofibrils (Vv(my,f] and mitochondria (Vv(mt,f] are respectively 76 and 9.5%. Beneath this layer are around 8000 red multiply innervated fibres. These have an average diameter of 25-40 micron. Vv(my,f) and Vv(mt,f) are 62 and 21% respectively, and M-lines are present. Around 11 000 white focally innervated twitch fibres lie beneath the red fibre zone. White fibres with an average diameter of 80-120 micron have a high Ca2+-activated myosin ATPase activity and Vv(my,f) and Vv(mt,f) are 78 and 5% respectively. Contractile properties of single skinned fibres were determined at 12 degrees C. Maximum Ca2+ activated tensions (kN m-2) and unloaded contraction speeds (muscle lengths s-1) were 49 and 0.5 for superficial, 70 and 1.4 for red and 180 and 4.4 for white muscle fibres. Superficial fibres have not been reported in other elasmobranchs with the exception of the closely related nursehound (Scyliorhinus stellaris L.) It is suggested that they are specialized for sustained force generation, having a tonic (postural) rather than a locomotor role.