Histochemical analysis was used to study the relationship between Rana temporaria tibialis anterior and lumbricalis IV fibre cross-sectional areas and concentrations of myosin ATPase and NADH reductase. Both tonic and twitch fibre types were histochemically identified in each muscle and the twitch fibres were subgrouped into types 1, 2, and 3. Fibres that had the largest cross-sectional areas were identified as the fibres which contained the highest myosin ATPase activity and the lowest NADH reductase activity (type 1 fibres). However, this relationship was more pronounced in the tibialis anterior muscle. In addition, single fibres from both muscles were isolated and injected with Ca2+ indicator aequorin. The fibres isolated from the tibialis anterior muscle were those with the largest cross-sectional areas relative to other fibres within a given muscle. The force responses and Ca2+ transients recorded from this group of single fibres were found to be fairly uniform, which may suggest that a single type of fibre was isolated. In contrast, the physiological properties of isolated lumbricalis IV fibres were highly variable and thus represented the responses of more than one fibre type.