The widespread occurrence of plant species confined to gypsum soils (gypsophiles) is noted in various semiarid and arid areas of the world. They are principally herbs and dwarf shrubs. Many of them contain very high levels of S and are relatively succulent, both on saline and nonsaline gypsum soil types. Possible reasons for the exclusion of some species from gypsum and for the evolution of gypsophily are discussed. Soil physical effects of gypsum may partly regulate the occurrence of widespread species tolerant of gypsum, but appear not to account adequately for the evolution of gypsophiles. Data on soil chemical effects of gypsum are inadequate and contradictory. It is possible that high levels of available soil S cause toxicity effects to some plants. Adaptations to this may include excretion of S by salt glands, uptake of large quantities of S and its dilution by succulence, partial exclusion of S and conversion of S into secondary organic compounds.