Slow vertical climbing and clinging are the dominant positional behaviors of the most convincing reconstruction of the primary spatial niche of Megaladapis, a giant extinct prosimian from Madagascar. The vertical support model of Cartmill ('74) predicts that clawless mammals should exhibit relatively elongated forelimbs in expanded size ranges. The allometric corollaries of this model are tested on closely related interspecific samples of Megaladapis and selected extant prosimians. Megaladapis and indriids (vertical leapers and clingers) conform to the structural predictions of the model, and are clearly distinguished from the more pronograde lemurids and cheirogaleids. Extreme hindlimb reduction (negative allometry) is coupled with moderate forelimb elongation (positive allometry) in Megaladapis. These body proportions effectively optimize pedal friction during vertical climbing and minimize the moment of body weight pulling the animal away from the trunk. Positive forelimb allometry occurs in the indriids, while isometry obtains for the hindlimb. The adaptive significance of these morphological strategies are discussed, as are possible selective mechanisms which effect the extreme hindlimb reduction in Megaladapis. Body weight estimates are also presented for Megaladapis edwardsi and Megaladapis grandidieri (50–100 kg and 40–75 kg, respectively).