Resistance in seedlings ofPinusarmandiiFranch. toCronartiumribicolaJ. C. Fisch. ex Rabenh. was high (ca. 72%). Resistance of artificially inoculated seedlings was expressed as: a percentage of seedlings lacking foliar or stem symptoms; a percentage of seedlings with foliar symptoms but lacking stem symptoms; or as a percentage of seedlings having bark reactions that were lethal to the fungus. Histological examinations indicated that the bark reactions resulted from necrosis of host cells which was apparently triggered by invading mycelium. Subsequent formation of a wound-periderm layer established a barrier between the diseased and healthy portions of the stem. Mechanisms providing resistance toC.ribicolainP.armandiiwere compared to such mechanisms inPinusmonticola(Dougl.) and other pines in response to infection by stem rusts; they were found to be strikingly similar.