The primary purpose of this study was to determine the extent of the variation in P. tremuloides and P. grandidentata with respect to several morphological and physiological characters. A knowledge of the natural variation of a given species should be the foundation for tree improvement work through selection and hybridization. The more complete understanding one has of the variability within a species, the more wisely one can organize and carry out a successful program of mass selection, individual tree selection, or a program of hybridization based on an intelligent choice of individuals.