The protein intermediates of the photoresponse and the modulation of this response in invertebrate photoreceptors are largely unknown. As a first step toward identifying these proteins, we have examined light-stimulated changes in protein phosphorylation in preparations of Limulus photoreceptors. Here we show that light modulates the level of phosphorylation of three proteins associated with Limulus ventral photoreceptors: the upper band of a 46-kD protein doublet (46A) and a 122-kD protein, which become more heavily phosphorylated in response to light, and the lower component of the 46-kD doublet (46B), which is phosphorylated in dark-adapted cells, but not in cells maintained in the light. In dark-adapted preparations, 46A is phosphorylated within 30 s after a flash of light and dephosphorylates over a period of many minutes. It is also a major substrate for calcium/calmodulin-dependent protein kinase (Wiebe et al., 1989); therefore, we speculate that 46A is involved in some aspect of dark adaptation. Interestingly, the level of phosphorylation of 46A is the same when measured from preparations maintained in complete darkness or ambient light for at least 1.5 h. The 122-kD phosphoprotein is the same protein which becomes phosphorylated in response to efferent innervation to Limulus eyes (Edwards et al., 1988) and the efferent neurotransmitter, octopamine (Edwards and Battelle, 1987). It may be involved in the increase in retinal sensitivity and the enhanced response of photoreceptors to light that is initiated by efferent innervation. Its role in light-stimulated processes is not clear. The level of phosphorylation of 46B may be most relevant to the long-term state of adaptation of the photoreceptor cell to light and dark.