An ecological survey was made of the vegetation of the region between 111 and 112[degree] W. Long. and 38 and 39[degree] N. Lat., the altitudinal range extending from the rock deserts of the Water Pocket Flexure at 5,000'' to the tops of the High Plateaus at 11,700''. Aquarius Plateau and the other High Plateaus are glaciated, lava-capped uplifts, while the Water Pocket Flexure and Miner''s Mt. are monoclines of sedimentary strata. Because of the edaphic and climatic variations in this region, its flora includes representative formations of all the zones from southern desert to alpine scrub. The lowlands are on the outskirts of Lower Sonoran territory whereas the summits are on the tension line between the subalpine forest and the arctic-alpine desert. The southern flora extends up the tributaries of the Colorado system; but corresponding levels on the Great Basin slopes of the tables lack these spp. The timber line seems to be determined by wind rather than by temp., for the broader the summit, the better the development of forest. Small tables and isolated peaks are occupied mostly by alpine scrub, larger tables, by park-like arrangements of scrub and forest. As far as possible the various successions to the climax in each climatic zone were worked out. A plant list of over 500 spp., with the ecological classification of each indicated, is included.