In many species of frog, the forelimb muscles important in amplexus are known to be much larger in males than in females. We studied this dimorphism in three forelimb muscles in the bullfrog [abductor indicus longus (AIL), flexor carpi radialis (FCR) and extensor carpi radialis (ECR)] by testing their isometric contractile properties. One muscle that is not dimorphic, the extensor carpi ulnaris (ECU), was also studied as a control. In addition to being greater in wet mass and in cross-sectional area in the males, our data show that the dimorphic muscles also produce significantly larger isometric forces in males than in females. The tetanic force per cm(2) of muscle cross-sectional area did not differ between the sexes, so that force within a muscle varies directly with muscle size. However, a number of the contractile variables we measured show that male muscles differ functionally from those of females. The male twitch contraction times were significantly longer in the AIL, and the male half-relaxation times were longer in both the AIL and FCR. These two dimorphic muscles were also significantly less fatiguable in males than were the corresponding female muscles. Their higher endurance resulted from the maintenance of high levels of unrelaxed force sustained between trains of stimuli during the fatigue test. This sustained force is much less pronounced in the female muscles, suggesting that high levels of sustained force may be a key functional feature that enables males to maintain amplexus economically for prolonged periods.