Between 1950 and 1955 hybrid progenies of lodgepole pine (Pinus contorta Dougl.) X jack pine (Pinus banksiana Lamb.) were tested to determine whether adaptation and performance in Montana and Idaho justified improvement of lodgepole pine by hybridization. Average heights, diameters, and survival rates of hybrids, of jack pines native to the Lake States, and of lodgepole pines native to Montana and Idaho were similar after 15 to 20 years of field tests. Poor growth and survival characterized progenies of California lodgepole pines, which represented the maternal parents of several hybrid groups. It was concluded that programs which rely on intraspecific variability are most feasible for the improvement of lodgepole pine in the northern Rocky Mountains.