Repetitive behaviors of six autistic children were observed under two conditions of background illumination. During two sessions, the room was illuminated by fluorescent light and during two other sessions, by equal intensity incandescent light. Subjects spent significantly more time engaged in repetitive behavior under fluorescent light. Previous research suggested that these findings were related to the flickering nature of fluorescent ilumination. Practical and theoretical implications were discussed. Further experimentation was suggested to assess relationships between flickering illumination and arousal.