The climbing behavior, muscle mechanics, and functional properties of selected forelimb muscles were examined to ascertain how three distantly related mammals may be adapted for climbing. To determine if features of the fox squirrel (Stalheim-Smith: J. Morphol. 180:55-68, '84) are general or unique features for a climber, two distantly related climbers, the raccoon (Procyon lotor) and the opossum (Didelphis virginiana), were studied. Muscle mechanics varied: the elbow flexors of the fox squirrel produced significantly more torque per unit mass than did the corresponding muscles of the opossum except at 80 degrees, but not more than the corresponding muscles of the raccoon. On the other hand, there were no statistically significant differences in torque per unit mass among the elbow extensors of the three climbers. Both elbow flexors and elbow extensor had faster contraction times and were more fatigable in the fox squirrel than in the opossum or in the raccoon. The data suggest that the musculoskeletal characteristics of the forelimbs of climbers vary according to behavioral, and possibly phylogenetic, differences.