During the month of February 1979, several hundred hatchling land iguanas (Conolophus pallidus) were observed emerging from their natal burrows in a 2 ha communal nesting area on Isla Santa Fe, Galapagos Islands. During this emergence, as many as nine Galapagos hawks were observed to patrol the nesting area and attack hatchling iguanas. The hypothesis that the ability of hatchling land iguanas to escape predation could be influenced by the interaction of the physiological state of the lizards and the thermal environment was analyzed using (1) empirical data on the effect of body temperature (Tb) on locomotory ability of iguanas and (2) biophysical modeling of the Tb's of hatchlings under natural conditions. This hypothesis was tested by assessing the success of natural hawk attacks on lizards exposed to different thermal environments. During those periods when predicted Tb's of hatchlings were always <32°C, (at which temperatures land iguanas were shown to have less than maximal ability to sprint rapidly) hawks were successful in 67% of the observed attacks. However, when Tb's of hatchlings were always ≧32° C, hawks were successful on only 19% of observed attacks. During periods when hatchling Tb's could be <32° C or 32-40° C (depending upon which microhabitat the hatchling occupied before the attack), the hawks were successful in 46% of the observed attacks. These data indicate that the physical environment, as mediated through the physiological state of the lizards and to correlated locomotary abilities, significantly affects the ability of hatchling land iguanas to escape predation. © 1981 Springer-Verlag.