Electron microscopy of the olfactory epithelium of two prosimian primates, the trees shrew and slow loris, and two simian primates, the macaque and gibbon, has shown that this epithelium consists of three cell types, receptor cells, supporting cells and basal cells, as in other mammals. Receptor cells were ciliated in all the animals investigated except the tree shrew, where, in addition to ciliated receptors, nonciliated receptors bearing only microvilli were occasionally present. Developing receptor cells containing numerous centrioles between nucleus and cell surface were occasionally observed and these cells had poorly developed olfactory knobs and few mitochondria. The olfactory epithelium was similar in morphology in all four species, except that supporting cells showed progressively more numerous, more slender and longer microvilli on their distal surface from tree shrew to slow loris to macaque to gibbon. This may imply a more discriminatory surface in the higher primates in view of the close relationship of these microvilli to the receptor cell surface.