A pandora moth (Coloradia pandora Blake) outbreak began in 1991 in a ponderosa pine (Pinus ponderosa Dougl. ex. Laws) spacing study area that also included scattered sugar pine (P. lambertiana Dougl). The relation of defoliation to five tree spacings (with and without understory vegetation) was examined, and stand growth reduction due to defoliation was estimated. Defoliation generally increased as spacing varied from 2 to 5.7 meters and then decreased as spacing increased to 8 meters. Partial defoliation in 1992 reduced stand volume growth, while partial defoliation in 1994 reduced height growth during 1990-94. Basal area growth of nonefoliated sugar pine and partially defoliated ponderosa pine trees outside the outbreak area. Ratios of annual basal area increments for nondefoliated trees to annual increments of partially defoliated ponderosa pine sharply increased after the outbreak. Basal area annual increments of sample trees were reduced by 25 prcent in the first growing season after defoliation (1992), 30 percent the second year after defoliation (1993), and 63 percent after the second defoliation (1994).