Three different types of single living muscle fibre were dissected from the iliofibularis muscle of Xenopus laevis. The fibres were mounted in a glass chamber and their rate of oxygen consumption was determined as a function of twitch frequency at 20 degrees C. The rate of oxygen consumption increased with twitch frequency until it levelled off and reached a maximum. The maximum rate of oxygen consumption varied between fibres (0.019 to 0.161 nmol O2 s-1 mm-3) and was reached at different twitch frequencies (less than 0.2 to 5.7 stimuli s-1). After the determination of the maximum rate of oxygen consumption, the succinate dehydrogenase activity in cross sections of the fibre was determined by means of a quantitative histochemical method. A proportional relationship between the maximum rate of oxygen consumption and the succinate dehydrogenase activity was found. The maximum rate of oxygen consumption and the succinate dehydrogenase activity are also proportional to the volume density of mitochondria in the three fibre types reported by Smith and Ovalle (1973; J. Anat., Lond. 116, 1-24). It is concluded that quantitative histochemistry of succinate dehydrogenase reliably predicts the maximum rate of oxygen consumption of muscle fibres in Xenopus laevis and that the maximum rate of oxygen consumption of single muscle fibres is determined by the volume density of mitochondria.