Among the endemic and sometimes localized genera of the Cruciferae occurring in South America, Mathewsia stands apart as an element of the distinctive desert flora of southwestern Peru and western Chile. As far as present records show, the genus is confined to a relatively narrow strip wholly west of the main Cordillera, not far from the Pacific Ocean. Some species are confined to washes or small arroyos known as quebrades. Others occur as part of the lomas vegetation of western Peru. Populations of the same species occupying different quebradas often differ from each other to a limited degree and in many instances probably little or no gene exchange occurs between them. The intervening dry areas, separating one quebrada from another, are completely unsuited to the growth of Mathewsia and form a natural barrier to the spread of any given population. Thus, with habitats only spottily available, the evolutionary divergence that has resulted in localized species of Mathewsia is readily understandable. One of the real difficulties in studying a group of species inhabiting areas that have been infrequently visited by botanists, is the paucity of available material. Furthermore, unlike some species of Cremolobus (Khanna and Rollins, 1965), the numbers of individuals in a given locality appear to be few and scattered. In some instances, only a single specimen of a given species has been found for study in all of the herbaria consulted. Under these circumstances, the bare essentials of the species are all that can be given and nothing can be said about their variation or distribution. The purpose of this paper is to present, as a first approximation, a taxonomic treatment of Mathewsia that will provide a sound basis for further research on the genus.