In order for vertebrate photoreceptors to exhibit their exquisite sensitivity that allows them to distinguish stimulation by a single photon, the sensor, rhodopsin, must have a very reproducible response. Rhodopsin propagates the signal from absorbed photons by activating transducin, but it is then inactivated by phosphorylation. Doan et al. measured the response of single rhodopsin molecules to single-photon absorption events in preparations of mouse photoreceptors. Multiple phosphorylation events provide independent inhibitory signals that together may provide the remarkable reproducibility of the amplitude and duration of rhodopsin activation observed in the vertebrate eye.