Many marine species spend part of their development in upper layers of the water column, where they may be exposed to solar ultraviolet radiation (UVR). For many of these species, light is one of the key environmental clues which triggers behaviorally-mediated adjustments in vertical distribution. We incubated planktonic larvae of the crab Cyrtograpsus altimanus in column-like aquaria to study their responses with/without UVR (under a solar simulator) and with/without a potential prey (the dinoflagellate Alexandrium tamarense). Their vertical distribution was recorded and used to evaluate the combined effects of UVR and the presence of the dinoflagellate on larval behavior. When UVR was absent, most larvae showed a tendency to swim upwards and to aggregate near the surface, regardless of the dinoflagellate presence. However, UVR inhibited this tendency and induced a repellent effect, which resulted in a more homogeneous vertical distribution of larvae. A. tamarense did not affect the vertical distribution of larvae. These results suggest that UVR-triggered, quick adjustments in vertical distribution might be an important strategy for C. altimanus larvae to cope with high solar radiation, which typically occur during the hatching season.