The spectral sensitivity of the deep-sea mysid, Gnathophausia ingens (Family Lophogastridae), was measured by electroretinography on intact specimens. High sensitivity to orange light was found. This was an unexpected result for a species whose adult members are never found above 400 m. Results of chromatic adaptation and silent substitution experiments were not compatible with either a one pigment or dual pigment visual system, making this one of the more unusual visual systems ever described.