Morphogenesis of photoreceptor outer segment disks appears to occur by an evagination of the ciliary plasma membrane (Steinberg et al., J Comp Neurol190: 501-519,'80). We tested if polymerized actin (F-actin) was necessary for the regulation of this postulated process by incubating Xenopus eyecups with 5 or 25 pM cytochalasin D for 6-28 hours. During the second hour, the incubation medium contained 3H-leucine. Both concentrations of cytochalasin resulted in: 1) dissolution of the rhodamine-phalloidin labeling pattern of photoreceptors, and 2) collapse of the calycal processes (which are normally filled with actin filaments) and disappearance of the inner segment microfilaments. In addition, the few most basal rod and cone outer segment disks appeared several times their normal diameter. These oversized disks had incorporated 3H-leucine and extended along the margin of the outer or inner segment. The nature of the overgrown disks is consistent only with a morphogenetic process involving evaginations of the ciliary plasma membrane. Deregulation by cytochalasin D was manifest by excessive growth of a few nascent disks rather than normal growth of many. Therefore, the normal network of actin filaments is apparently not necessary for continued evagination of the membrane, but it does seem to be an essential part of the mechanism that initiates the evagination of the ciliary plasma membrane andlor the mechanism that controls how far nascent disks grow.