The hypothesis that differences in woodland and grassland rodent fauna were responsible for the absence of range caterpillar,Hemileuca oliviaeCockerell, in woodlands adjacent to infested grasslands was tested by field and laboratory experiments. There was little predation by any species on range caterpillar larvae. Three species exhibited active predation on eggs and pupae. Two of these species,Peromyscus maniculatus(Wagner) andOnychomys leucogaster(Wied-Neuwied), were found in grasslands inhabited by the range caterpillar. The third species,Peromyscus truei(Schufelt), was common in woodlands and showed particularly avid predation of range caterpillar pupae. The greater density and predation rate of this species over the grassland species was probably an important factor limiting the survival of range caterpillar in woodlands even when favorable host plants were present.