Iguanid lizards exhibit considerable intraspecific variation in several aspects of their muscle composition. To determine the relationship of this variation to the variation in locomotor performance, running speeds of 20 male desert iguanas (Dipsosaurus dorsalis) of similar mass were measured from video recordings of animals as they sprinted down a 4.9-m runway maintained at 40 degrees C, the preferred body temperature of Dipsosaurus. Mean sprint speed ranged from 2.2 to 4.2 m/s. Selected muscles from these animals were then analyzed histochemically for fiber type size and composition, and the activities of citrate synthase, pyruvate kinase, and creatine kinase were measured. Muscle fiber cross-sectional areas were highly correlated within individuals, in three leg muscles and across all three fiber types, so that individuals could be characterized as possessing large or small fibers relative to the sample mean. Activities of all three enzymes also covaried within individuals so that individual lizards could be characterized as possessing high or low leg muscle catabolic capacity. There existed a significant and inverse relationship between fiber cross-sectional areas and muscle enzyme activities so that individuals with small muscle fibers tended to have higher catabolic capacities. Approximately 25-30% of the variation in mean sprint running speed could be predicted by variation in muscle fiber areas alone. The use of muscle fiber areas and snout vent length as independent variables in a multiple-regression equation explained approximately 50% of the sprint-running variation.(ABSTRACT TRUNCATED AT 250 WORDS)