The broad-leaved evergreen shrub Simmondsia chinensis has a wide ecological range of distribution. Populations are found in Mexico, southern California, and Arizona, in the deserts as well as in mesic coastal habitats. Plants of this species possess distinctive characteristics in their structure and distribution which offer excellent material for the study of adaptation to the desert environment. A seasonal study of the physiological ecology of these plants was made in both coastal (San Diego) and desert habitats (Morongo Canyon, Twentynine Palms, Tucson) to investigate the differential environmental responses of these populations. The seasonal physiological measurements recorded included leaf water potential, net photosynthesis, dark respiration, and carbohydrate content. Also, synecological studies were conducted at the four study sites. Comparative laboratory experiments were made to study the effect of certain environmental factors, such as water availability and temperature, on the metabolism of plants of these populations which were grown in a common environment. The seasonal responses of plants from these populations in diverse climates indicated distinctive physiological characteristics of the plants of the coastal and desert habitats which corresponded to the climatic conditions during the study period at these respective sites. In all habitats, however, including coastal and severe desert, the plants of the four populations were physiologically active during the entire year of study, indicative that these plants are true drought endurers and have the capacity to maintain a positive carbon balance even under conditions of severe desert drought. Seasonal metabolic activities were generally correlated with leaf water potential variations. These correlations were greater in the desert plants than in the coastal ones. Also, the seasonal leaf water potential differences were greater in the desert plants of the Morongo Canyon study site than in the coastal San Diego plants. A comparative laboratory study of the physiological ecology of plants of the different populations showed that all possessed a high degree of physiological tolerance to drought and temperature; however, there were distinctive differences between the coastal and desert populations which corresponded with the field observations. The results of the field and laboratory studies indicate that populations of S. chinensis have increasing adaptations to drought which relate to decreasing available moisture in their natural environments.