Distinctive linear arrays of intramembrane particles were present in microvillar membranes of approximately 5% of surface columnar cells observed in freeze-fracture replicas of monkey colon and human rectum. On these cells, longitudinally-oriented rows of P face particles and corresponding E face grooves appeared on all exposed microvilli. The constituent particles varied from round (8-9 nm in diameter) to rod-shaped (18 nm long). Microvilli of the great majority of columnar cells displayed randomly distributed P face particles similar to those previously observed in small and large intestine of birds and small mammals. The significance of the linear arrays is not known. It is postulated that they may represent protein assemblies which are specific to a functionally-distinct subpopulation of primate intestinal columnar cells.