Ecotypic specialization in different parts of the ranges of widely distributed species is a phenomenon that has commanded the attention of botanists for several decades. Experience has usually shown that when plants are moved from their native habitat into a different environment they become subject to injuries from factors associated with the new environment, although occasionally they may prove more successful. The slow genetic fitting of plant populations to the peculiarities of the area which they have occupied for a long time is especially important in the artificial establishment of forests, for usually there are available for use in plantations seed lots representing the same species but from populations attuned to widely different environments. The long life-cycle of the tree makes it possible to invest much time and money in reforestation before genetic weakness becomes apparent; moreover, the unsuccessful planting may be expensive to remove, and by cross pollination it may seriously contaminate near-by native trees. In recognition of this problem the United States Forest Service established an experimental plantation of Pinus ponderosa Laws. in northern Idaho, with the purpose of determining the relative value of seeds from different parts of its area for use in reforestation in northern Idaho.