The cold tolerance of Cylas formicarius from five islands in the southwest of Japan was studied. The thermal threshold of the reproductive activity of adults tended to be lower and the supercooling point of the pupae was a little higher for the Okinawa-Honto population than for the others. The cold tolerance of the adult weevils was slightly accelerated by cold acclimation to 15°C for 4 d. Reproductive diapause induced under 10L : 14D increased cold tolerance significantly. Therefore, the cold tolerance of adult weevils would be accelerated by both cold acclimation at decreasing temperatures from autumn to winter and reproductive diapause induced by short daytime hours. When considering the increased cold-tolerence of C. formicarius through such mechanisms together with the temperature conditions in winter at the northern edge of its distribution area in the USA, C. formicarius probably has the capability to survive in the coastal zone of the Pacific Ocean in southern and central Japan.