The forest tent caterpillar, Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae), is distributed throughout North American hardwood forests. Although considered polyphagous, regional populations tend to use only a few host species for oviposition, suggesting that M. disstria is more oligophagous than commonly thought. We tested this premise using larvae from Manitoba, Canada; Michigan; and Louisiana in a factorial, reciprocal transplant experiment. Pupal mass, development time, and survival were recorded for each population after rearing larvae in Louisiana on three primary hosts used by southern populations (water tupelo, Nyssa aquatica L.; sweetgum, Liquidambar styraciflua L.; and water oak, Quercus nigra L.) and in Michigan on three northern host trees (red oak, Quercus rubra L.; trembling aspen, Populus tremuloides Michaux; and sugar maple, Acer saccharum Marshall). Manitoba, Canada and Michigan populations had the highest pupal mass, best survival, and most rapid development on trembling aspen and red oak, both northern species. Louisiana larvae attained the highest pupal mass on water tupelo, a primary host across the Gulf States. Northern populations grew poorly on water tupelo, whereas Louisiana caterpillars had the smallest pupal mass and poorest survival on sugar maple. Both red and water oaks were acceptable hosts for all three populations. Our results indicate that M. disstria is actually a composite of regionally adapted populations rather than an extreme generalist. It is unclear, however, what mechanisms might reduce gene flow, allowing such specialization to evolve and persist. We suggest that varying phenology in adult flight times among populations feeding on different hosts could provide at least a partially isolating mechanism, allowing for the evolution of host adaptation.