Evidence is given in support of the hypothesis that the spatial arrangement of attacks by the mountain pine beetle (Dendroctonus ponderosae Hopk.) on its host, lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.), is largely determined by the pattern of bark niches suitable for attack initiation. Density of attack sites and bark thickness are important in determining the attack-harbouring potential of the bark and in determining the upper limit of attacks, thus, the potential area available for attack on the bark surface of lodgepole pine trees. At fixed levels on the clear bole of naturally infested trees, attack pattern tends to be regular owing to the regular spatial pattern of suitable attack sites. The regularity of the attack pattern is disturbed by the presence of an attack-density gradient around the stem circumference. The vertical attack-density gradient over the host is well described in terms of a relation between attack density and bark thickness. This relation is asymptotic and rests on the assumption that the density of bark niches suitable for attack initiation approaches an upper limit with increasing bark thickness on lodgepole pine trees.