Both the fast and slow muscle fibres of advanced teleost fish are multiply innervated. The fraction of slow-fibre volume occupied by mitochondria is 31.3%, 25.5%, and 24.6%, respectively, for the myotomal muscles of brook trout (Salvelinus fontinalis), crucian carp (Carassius carassius), and plaice (Pleuronectes platessa), respectively. The corresponding figures for the fast muscles of these species are 9.3%, 4.6% and 2.0%, respectively. Cytochrome-oxidase and citrate-synthetase activities in the fast muscles of 9 species of teleost range from 0.20-0.93 mumoles substrate utilised, g wet weight muscle(-1) min(-1) (at 15 degrees C) or around 4-17% of that of the corresponding slow fibres. Ultrastructural analyses reveal a marked heterogeneity within the fast-fibre population. For example, the fraction of fibres with less than 1% or greater than 10% mitochondria is 0, 4, 42% and 36, 12 and 0%, respectively, for trout, carp and plaice. In general, small fibres (less than 500 micrometers(2)) have the highest and large fibres (greater than 1,500 micrometers(2)) the lowest mitochondrial densities. The complexity of mitochondrial cristae is reduced in fast compared to slow fibres. Hexokinase activities range from 0.4-2.5 in slow and from 0.08-0.7 mumoles, g wet weight(-1) min(-1) in fast muscles, indicating a wide variation in their capacity for aerobic glucose utilisation. Phosphofructokinase activities are 1.2 to 3.6 times higher in fast than slow muscles indicating a greater glycolytic potential. Lactate dehydrogenase activities are not correlated with either the predicted anaerobic scopes for activity or the anoxic tolerances of the species studied. The results indicate a considerable variation in the aerobic capacities and principal fuels supporting activity among the fast muscles of different species. Brook trout and crucian carp are known to recruit fast fibres at low swimming speeds. For these species the aerobic potential of the fast muscle is probably sufficient to meet the energy requirements of slow swimming.