The combined influence of plant competition and defoliation on development of Canada thistle was investigated in a 2-yr field study. Plant competition was induced by seeding tall fescue and crown vetch. Artificial defoliation was used to simulate various levels of leaf removal by insects. Both defoliation and induced competition reduced biomass of Canada thistle but their impact varied with environmental conditions. Defoliation had a greater detrimental influence than induced competition on thistle biomass in a dry year when growth of the plant competitors was suppressed. In a wet year, induced competition was more important in suppressing Canada thistle than defoliation, and moderate levels of defoliation (25%), applied once when the thistles were 12 to 15 cm in diam, stimulated root weight within the top 20 cm of soil. Reduction in thistle biomass increased with level of defoliation and was greatest when defoliation was applied repeatedly at 14-d intervals in the presence of induced competition. Crown vetch showed very little growth in one season and tall fescue was the primary source of competition for the thistles. The results confirm the hypothesis that combined stresses can substantially reduce biomass development of Canada thistle plants.