Choroid plexus and intestinal microvilli in thin sections have microfilaments in the cytoplasm adjacent to the membranes, and in replicas have broken strands of filaments in both cytoplasm and on E faces of plasm membranes. The microfilaments contain actin as indicated by their binding of heavy meromyosin (HMM). In sections of choroid plexus, the microfilaments are 7-8 nm in diameter and form a loose meshwork which lies parallel to the membrane and which is connected to the membranes both by short, connecting filaments (8 times 30 nm) and dense globules (approximately 15-20 nm). The filamentous strands seen in replicas are approximately 8 nm in diameter. Because they are similar in diameter and are connected to the membrane, these filamentous strands seen in replicas apparently represent the connecting structures, portions of the microfilaments, or both. The filamentous strands attached to the membrane are usually associated with the E face and appear to be pulled through the P half-membrane. In replicas of intestinal brush border microvilli, the connecting strands attaching core microfilaments to the membrane are readily visualized. In contrast, regions of attachment of core microfilaments to dense material at the tips of microvilli are associated with few particles on P faces and with few filamentous strands on the E faces of the membranes. Freeze-fracture replicas suggest a morphologically similar type of connecting strand attachment for microfilament-membrane binding in both choroid plexus and intestinal microvilli, despite the lack of a prominent core bundle of microfilaments in choroid plexus microvilli.