The transition of behavioral states of medaka fish in response to exposure to light was investigated by using an automatic tracking system with an image processor. The movement tracks were recorded in every 0.25 sec for 2 hours. Spatio-temporal variation in the movement tracks was analyzed as time progressed after the light signal was changed. The pattern of “alert” appeared through transition of movement behavior. Cross-correlations on locations revealed changes in behavioral states. Additionally, the movement tracks were segmented for 1-2 minutes, and were classified by the self-organized mapping (SOM) based on speed, meander, angle changes, etc. The grouping on the SOM correspondingly characterized the movement patterns before and after the light exposure.