The visual pigments of four mesopelagic crustacean species were studied at sea by means of microspectrophotometry. The absorbance maxima obtained for the visual pigments and their metarhodopsins, respectively, were: 493 nm and 481 nm (Systellaspis debilis), 485 nm and 480 nm (Acanthephyra curtirostris), 491 nm and 482 nm (A. smithi), and 495 nm and 487 nm (Sergestes tenuiremis). The spectral characteristics of the rhodopsins and metarhodopsins permit high photosensitivity and facilitate photoregeneration in a nearly monochromatic environment. Photic regeneration of rhodopsins from the deep-sea environment was demonstrated, and data were obtained which are consistent with the occurrence of dark regeneration. Specific optical density of the observed visual pigments was calculated for two species.