When individuals of the crustacean Mysis relicta are fully dark-adapted and preparations for electroretinogramme recordings are performed in infra-red light of wavelength greater than 730 nm, responses to 300 ms flashes of light can be recorded. Though this is the first time that responses to such long wavelengths have ever been reported for any invertebrate photoreceptor, the authors suggest that near infra-red sensitivity in arthropods could be more widespread, and that because of very long adaptation and sensitivity regeneration times preparations should not be carried out in red light, hitherto deemed invisible to most arthropods, but with the aid of an infra-red image converter and a filter at wavelengths greater than 800 nm.