The major soluble protein of the isolated brush-border of the intestinal epithelium has a molecular weight and net charge indistinguishable from those of skeletal-muscle actin, as determined by polyacrylamide gel electrophoresis. Furthermore, this protein, isolated from acetone powders of the purified brush-border, undergoes a G to F transformation in the presence of Mg(++). The filaments have a substructure indistinguishable from muscle actin, as seen by the negative-staining technique, and bind heavy meromyosin with the arrowhead configuration characteristic of actin. The filaments in the microvilli of the intact bruch-border also bind heavy meromyosin. Thus, actin seems to be present in intestinal epithelial cells.