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Scorpions are nocturnal arachnids that fluoresce a bright cyan-green when exposed to UV light. Although the function of this fluorescence remains unknown, some authors have suggested that it may aid the scorpions’ light detection. Taking advantage of scorpions’ negatively phototactic behavior, we tested the responses of desert grassland scorpions, Paruroctonus utahensis (Williams 1968), to 395 nm UV light at irradiances corresponding to an hour before sunset (0.15 mu W/cm(2)), sunset (0.01 mu W/cm(2)), and moonlight (0.0001 mu W/cm(2)), as well as no light. We found that animals showed the strongest responses to UV light levels equivalent to sunset. The animals moved more quickly and sporadically under the higher light levels. In addition, animals were less likely to complete a trial under highest light conditions, suggesting that UV light may inhibit normal scorpion locomotion. Finally, this study resulted in several methodological refinements, including automated tracking of the subjects’ movements that should prove useful in future behavioral studies of scorpion phototactic behavior.