The xylem resin of more than 5,700 Pinus ponderosa Laws. trees at 68 locations in Western United States was analyzed in a 9-year study. The quantity of the five major components-a-pinene, ,8-pinene, 3-carene, myrene, limonene-of the monoterpene portion of individual trees varied widely over the range of the species. This range can be conveniently divided into five regions and four transition zones on the basis of plot averages for the five components. A working genetic hypothesis is proposed for classifying individual tree types based on monoterpene composition; liS types were found. One of the rare composition types, high in limonene, may prove to be resistant to attacks by the western pine beetle (DefidroclOllus brevicomis Lec.), a major pest of ponderosa pine. Results are also reported on: quantity of resin flow; percent terpene in whole resin; nongenetic influence on qualitative composition; unusual amounts of minor rnonoterpene components; amount of sesquiterpene; association of monoterpene composition with resin color, bark color, and needles per fascicle; association of resin quantity and quality with bark beetle distribution, abundance and host resistance.