The lipophilic extractives, that is, wood resin, in Populus grandidentata (bigtooth aspen) and Populus tremuloides (quaking aspen) sapwood, heartwood, and knots were analyzed. The amount and composition of the extractives in sapwood were similar in both species. Triglycerides, short-chain fatty acids, and steryl esters were predominant in sapwood samples. The knots of both species contained similar amounts of extractives as the stemwood, but the composition was quite different. Monoglycerides; long-chain fatty acids and alcohols, with 22 to 28 carbon atoms; hydroxy fatty acids; and ferulic acid esters of fatty alcohols were the major components in heartwood and knots, but were minor components in the sapwood.