Several species of deep‐sea crustaceans possess unusually high spectral sensitivity to UV light, measured elec‐trophysiologically. In addition, behavioral experiments indicate that these species are equally sensitive to near‐UV and blue‐green light. These results raised questions about the functional significance of this short wavelength sensitivity, since it is commonly believed that UV light is virtually absent in the deep‐sea. Using submersible based technology, we conducted measurements of downwelling irradiance at two wavelengths ‐ 380 and 480 nm. These measurements indicate that the irradiance of 380 nm light at 500–600 m, the daytime depth range of the crustaceans with a U V sensitivity peak, is high enough to be detected by these species. In addition, measurements of the spectrum of the downwelling light field conducted at dusk demonstrate that spectral changes visible at the surface are not visible at 150 m. These results are discussed with respect to hypotheses on the functional significance of UV vision in deep‐sea crustaceans.