In recent years a number of cases have been described of plant species which grow together, bloom at the same time, are more or less interfertile, yet do not hybridize freely in nature because of mechanical isolation. Examples are found in such genera as Aquilegia, Sarcostemma, Penstemon, Mimulus, Pedicularis, Ophrys, and Stanhopea (these cases have been reviewed by Grant [1963, p. 355-359] and will be discussed in a later section of this paper). In each case the species involved possess complex floral mechanisms adapted to a relatively narrow range of different animal pollinators. These mechanical differences, operating at the stage of pollination, serve to partially or wholly exclude pollinators other than those to which each plant species is adapted (Grant, 1949, 1963). Salvia apiana and S. mellifera (section Audibertia) are two closely related and widespread perennial species of sage in the California sage community. This pair of species, and the breeding relationships between them, have been studied by Epling (1947a, b) and by Anderson and Anderson (1954), and some aspects of the case are -therefore well known to plant evolutionists now. Although the two interfertile species occur sympatrically over a wide area in southern California, natural hybridization is restricted by a combination of isolating mechanisms. However, heretofore, the part played by mechanical isolation in the separation of the two species has not been clarified. In particular, the role of the respective insect pollinators of S. apiana and S. mellifera in reproductive isolation has not been understood. The purpose of the present paper is to describe the mechanical isolation existing between S. apiana and S. mellifera and some consequences of the habits of their insect pollinators. The present study based on observations of pollinators made since 1959 thus sheds new light on some aspects of the relationship between S. apiana and S. mellifera, as well as adding another case to the theory of mechanical isolation in angiosperms.