Intensities of histochemical and immunohistochemical reactions in muscle fibres of Rana and Xenopus have been estimated microphotometrically, and the data from serial sections statically analysed. Quantitative validities of reactions and measurements have also been assessed against independent published evidence. It is concluded that NADH-tetrazolium reductase overestimates tonic-fibre aerobic capacities and the actomyosin ATPase reaction overestimates their contraction speeds. However, it appears that succinate dehydrogenase, despite being a near-equilibrium enzyme of particulate distribution, indicates the relative aerobic capacities of fibres with acceptable accuracy when lightly reacted. Capacities for aerobic and anaerobic metabolism are positively correlated over all types of fibre (r typically approximately 0.6 for 200 fibres), perhaps as an adaptation to environmental hypoxia. Multivariate clusters (indicating fibre types) have been sought, using Ward's method with optimizing procedures (iterative relocation and multivariate-normal modelling). Cluster analysis confirms the subjective identifications of two 'slow/tonic' types in Xenopus (labelled T5 and S4) but of only one (T5) in Rana. Division of the 'fast family' twitch fibres into three types (F1-F3) in both genera, with metabolic capacity related inversely to apparent shortening velocity, is highly supportable by objective criteria. However, statistically significant subdivisions also present themselves. Rana F2 and Xenopus F1 clusters can be bisected according to metabolic capacity; and Xenopus F2 fibres fall into three subtypes reflecting different isomyosin contents. In the different types of twitch fibre, ratios of myofibrillar ATP consumption rate to aerobic capacity increase up to 30-fold with contraction speed, but anaerobic/aerobic ratios do so only 5-fold.