The spectral sensitivity of receptor cells in the eye of the spiny lobster Panulirus argus was measured with intracellular electrodes. All cells sampled were maximally sensitive at about 510 nm. Several of these retinular cells were filled with Lucifer Yellow-CH from the recording electrode, and subsequent histological examination showed that they contributed to the main rhabdom and their axons terminated in the lamina ganglionaris. Microspectrophotometry of the main rhabdom revealed a single visual pigment with λmax at about 510 nm. Using extracellular recording and selective adaptation with colored lights, however, a second receptor type was unmasked, with peak sensitivity at 370 nm in the UV. Indirect evidence suggests that it is the small, eighth retinular cell located distal to the main rhabdom, which has been shown in crayfish to contain a visual pigment maximally sensitive at short wavelengths and to make a comparably small contribution to the ERG.