Isolated retinal precursor cells, grown without pigment epithelial or glial cells and in the absence of intercellular contacts, develop a complex set of photoreceptor-specific properties, including polarized structural and molecular organization and opsin immunoreactivity. We report here that these isolated embryonic photoreceptors are also capable of responding to light. Sequential photography showed that 50% of the photoreceptors grown in a light cycle elongate when exposed to light and contract in response to darkness. A smaller population (20%) showed the opposite response. Responses of individual cells could be observed during several sequential light cycles and resemble photomechanical movements in vivo [Ali, M. A. (1971) Vision Res. 11, 1225-1288]. The differentiation program expressed by isolated precursor cells, therefore, includes the capacity for highly complex functional activities that require light sensitivity. These observations raise challenging questions regarding the nature of the chromophore and pigments that mediate light-regulated behaviors of cultured photoreceptors.