1. Receptor potentials in honeybee drone retinula cells were recorded with intracellular micro-electrodes in the dorsal part of the superfused retina. The light stimuli were sufficiently weak that the response amplitude was proportional to the intensity. 2. Responses to stimuli of different wave-lengths, although of different amplitude, all had the same time course. 3. The maximal sensitivity in all the cells recorded from was to a wave-length between 450 and 460 nm. 4. Microspectrophotometry showed the presence of a pigment with two stable states, interconvertible by light, absorbing maximally at 445 nm (rhodopsin) and 505 nm (metarhodopsin). 5. There was a good match between the absorption spectrum of rhodopsin and the spectral sensitivity of retinula cells. 6. Transformation of a large fraction of rhodopsin to metarhodopsin by light reduced the sensitivity of the retinula cell but did not alter the shape of the relative spectral sensitivity curve or the time course of the responses. 7. It is concluded that for weak lights the receptor potential is determined only by the number of rhodopsin molecules that absorb photons: neither the presence of metarhodopsin nor its phototransformation to rhodopsin produces a detectable effect.