Behavioral responses to environmental factors at the pelagic larval stage can have a crucial effect on habitat selection and therefore adult distribution in many benthic organisms. Reef-building scleractinian corals form sessile colonies in distinct light environments, and it has long been suggested that the light response of free-swimming larvae affects the formation of the light-dependent distribution. However, due to the lack of real-time observations of larval behavior, the characteristics of immediate response to variable light stimuli remains to be clarified in corals. Here, we analyzed the swimming behavior of larvae of the common reef coral Acropora tenuis under various lighting conditions and found that they exhibited a step-down photophobic response, i.e. a decrease of swimming speed in response to a rapid attenuation of light intensity. Observations of larvae under different wavelengths indicated that only the loss of blue light produced a significant response whereas the magnitude of the response with other wavelengths was relatively small. Mathematical simulations suggest that the photophobic response causes aggregation of larvae in the lighter areas of a two-dimensional rectangular field. These results suggest that the step-down photophobic response of coral larvae has an important role in the forming of the light-dependent distribution of adult colonies.