The rice leaf folder, Cnaphalocrocis medinalis, showed a circadian rhythm in its calling behavior at 25 ± 1 oC. Female moths showed calling behavior most frequently 5-7 h into the scotophase under LD 15:9. Female moths at age 4 (days), after entraining at LD 15:9, showed a free-running rhythm for at least two more cycles in continuous darkness. The rhythm was damped out in constant light. The temporal pattern in the calling behavior was affected by different LD cycles (LD 15:9, 12:12, and 9:15), indicating that the rhythm in calling behavior was entrained by the LD cycles. By changing one photoregime condition to another after calling had been initiated, it was shown that C. medinalis females use the light-off signal to phase-set the circadian rhythm of calling behavior. Our results suggest that the rhythm in calling behavior is a circadian rhythm entrained by LD cycles.