The growth performance of selected Pinus contorta and Pinus ponderosa seed sources in the greenhouse was differentially affected by inoculation with three different ectomycorrhizal fungi. Pinus contorta seedlings exhibited high levels of infection which were nonlinearly and positively correlated with total dry weight for Suillus granulatus and Pisolithus tinctorius fungal treatments. Ectomycorrhiza formation in these two fungal treatments was nonlinearly and negatively correlated with P. contorta root/shoot ratio. Pinus ponderosa seedlings possessed low levels of infection in all ectomycorrhizal fungus treatments and showed no correlation between infection and dry weight. The benefit seedlings received from ectomycorrhiza formation was controlled directly by the amount of infection rather than the host genotype. Host genotype had an indirect effect by influencing the amount of ectomycorrhizae formed. Forest Sci. 28:237-250.