A tension-sensitive microfilament (MF) system in the plane of zonula adherens (ZA) junctions of retinal pigment epithelial (RPE) cells is described. Organ cultures and whole mounts of RPE from chick embryos were labeled for F-actin and examined with a confocal laser scanning microscope. The changes in the distribution of MFs during alternations in the direction of tensional stress, generated in the organ culture system or induced by mechanical wounding, were examined. In RPE cells throughout the organ culture, fine MF bundles, termed apical stress fibers (ASF), oriented parallel to the direction in which tension had been generated, were observed at the level of circumferential MF bundles (CMBs) associated with ZA junctions. Within 24 h after the direction of the tension was experimentally altered by 90 degrees, the orientation of ASF also changed, becoming aligned along the new lines of the tension. The ASF likewise react to changes in the direction of stress generated when RPE cells respond to wounding in organ culture. The presence of similar MFs in RPE cells in the posterior retina of 13-day chick embryos in situ may reflect mechanical stretching of the RPE cells that occurs during the development of the eye. The ASF probably limit the stretching of the CMBs of RPE cells in any one direction, thereby protecting the RPE cells against tensional stress. This system of MFs may be especially important in chick RPE cells, which lack desmosomes and keratin-type intermediate filaments that normally have this function in other cell types.