Humans cannot see ultraviolet light, but many arthropods and vertebrates can because they have a single photo-receptor with a peak sensitivity to light at wavelengths of around 350 nanometres (ref. 1). Here we use electrophysiological methods to investigate the vision of the mantis shrimp, Neogonodactylus oerstedii. We find that this marine crustacean has at least four types of photoreceptor for ultraviolet light that are located in cells of the eye known as R8 cells. These photoreceptors are maximally sensitive to light of wavelengths 315, 330, 340 and 380 nm. Together with previous evidence, this finding indicates that the remarkable colour-vision system in these stomatopod crustaceans may be unique, as befits their habitat of kaleidoscopically colourful tropical coral reefs.