The third ventricle of monkeys has been examined with the scanning electron microscope (SEM). Two populations of supraependymal (SE) cells were distinguished on the basis of morphology and location. One type has been previously reported (Coates, '72, '73a,b,c). Another, type 2 SE cell, is now described. Type 2 SE cells were found in the third ventricle of both sexes and in all age groups although the numbers varied highly from animal to animal. The most common site for type 2 SE cells was the floor and transition zone of the third ventricle. Visualized with SEM, these cells had variable morphology, but may be characterized by a small cell body, few non-branching processes some of which were flared and surface features such as ruffled membranes. Type 2 SE cells most likely correspond to Kolmer or epiplexus cells originally described in association with the choroid plexus. As such, they are probably phagocytes.