The different geographic populations of Lophocampa maculata Harris 1841 are characterized by a variety of phenotypic differences, of which larval color is the most obvious. Individuals of the Pacific Northwest (PNW) populations display significant variation in late instar coloration, arising from variation in setae pigmentation. Such variation is not found in other populations of Lophocampa maculata, including the western Interior population (WI) and California Coastal population (CAC). Analysis of the pattern of pigmentation of the PNW population suggests it represents a combination of features of the CAC and WI populations. A simple scheme that accounts for all of the color variations seen in the different geographic populations of L. maculata is presented. A laboratory mating experiment involving WI and CAC individuals resulted in viable offspring displaying the range of larval coloration seen in the wild PNW populations. The F1 hybrids were fertile and produced an F2 generation also exhibiting the PNW larval color patterns. Taken together, these results suggest that the PNW populations arose via hybridization between the adjacent WI and CAC populations. Evidence from laboratory-raised broods of wild-caught females suggests there can be significant individual variation in pigmentation even within a single brood. The present day PNW populations demonstrate features of a hybrid swarm resulting from relatively recent hybridization. A model for this process since the last glacial maximum is presented.