The suitability of Garry oak (Quercus garryana) and red alder (Alnus rubra) as hosts for the gypsy moth,Lymantria dispar(L.), was assessed under laboratory conditions by observing larval survival, larval weights, foliage consumption, developmental period, pupal weight, and ova production. Survival was not significantly different between larvae fed Garry oak (98.7%) or red alder (97.4%). The mean maximum live larval weights were significantly different between sexes but not between diets. Females weighed 2,498 mg when fed Garry oak and 2,210 mg when fed red alder. Males weighed 894 mg when fed Garry oak and 737 mg when fed red alder. The mean amount of foliage consumed was significantly different between sexes but not diet. Female larvae consumed an average of 705 cm2of Garry oak and 678 cm2of red alder foliage. Male larvae consumed 247 cm2of Garry oak and 253 cm2of red alder foliage. The mean time from egg hatch to pupation was significantly different between sexes and diets. Female larvae pupated in 39.5 d on Garry oak and 48.1 d on red alder. Male larvae pupated in 33.4 d on Garry oak and 40.6 d on red alder. Pupal weights were not significantly different between sexes or diets. Male pupae weighed 554 mg from a larval diet of Garry oak and 572 mg from red alder. Female pupae weighed 1,846 mg from Garry oak and 1,711 mg from red alder. An average of 863 ova (Garry oak) versus 758 ova (red alder) was present in the reproductive tract of 2-d-old females, an insignificant difference. Pupal weights, frass production, and ova production were highly correlated. Nutritional indices indicated that Garry oak foliage was converted into biomass slightly more efficiently than that of red alder. These data indicated that foliage of either Garry oak or red alder provided a very suitable diet for the gypsy moth.