The ability of halophytes to develop under conditions which would mean death to most vegetation is well known. Salt marsh plants, growing where sodium chloride is the chief constituent of the soil solution and where salt concentrations usually vary from 1 to 6 per cent., are the most common halophytes. Much less numerous than the salt marshes are the lakes whose chief constituent is magnesium sulphate, a compound toxic to many plants in any but very dilute concentrations. Although the plants of the two types of lakes are to some extent the same, very little work has been done on those of the magnesium sulphate lakes. In view of the common toxicity of magnesium salts to plants and the scarcity of information concerning the physiological relations of halophytes, it has seemed worth while to investigate the vegetation of certain lakes lying in north central Washington and adjacent British Columbia which are unusual in having a very high proportion of magnesium sulphate in the salt content and in supporting seed plants at salt concentrations as high as 25 or 26 per cent. Studies were made on three lakes lying in the eastern foothills of the Cascade Mountains in the northern part of Okanogan County, Washington, and British Columbia, in one of which Buppia maritima L. grows abundantly, completely submerged in the salty water of the lake. The first of these, known locally as Epsom Lake and designated in this paper as no. 1, lies about 4 miles northwest of the town of Oroville, in northern Washington, about midway between Lake Osoyoos and the Similkameen River. The geology of the region was described by Jenkins (10). St. John and Courtney (35) listed the phanerogams growing in the lake and on the walls of the surrounding basin. They report the banks to be of practically pure salt, in which the plants were apparently rooted. Analyses of salts taken from the surface of the water gave a content of 99.64 per cent, of MgSO4. The lake lies at an elevation of about 2000 feet, with walls of the basin reaching several hundred feet higher. The lake itself, which drains an area of about one square mile, is, when full, approximately four acres in extent and 30 feet deep. There is no outlet and drainage waters remain until evaporated during the dry summer season, when the lake is reduced to a series of small i Contribution no. 39 from the Botany Department of the State College of Washington.