In the past, it has been proposed that basilisk lizards (Basiliscus basiliscus) are able to reach high population densities because the juveniles and adults have differing water-running abilities and therefore live in different habitats. However, there is no a priori reason to expect juveniles to be better able to run on water than adults. To determine the causal relationship between body size and water-running ability, we made three types of measurements: (1) direct morphological measurements on preserved specimens; (2) hydrodynamic measurements on physical models of the lizards' feet; and (3) kinematic measurements on basilisk lizards running on water. The information gathered from these investigations was used to develop an allometric model which predicted the maximum upward force impulses that the lizards could generate. We find that small lizards have a capacity to generate large force surpluses. A 2 g lizard can generate a maximum upward impulse that is more than twice that needed to support its body weight (225 %). In contrast, a 200 g lizard, under optimal conditions, can just barely support its body weight (111 %).