Geographic variation of photoperiodic response in larval development of the yellow-spotted longicorn beetle, Psacothea hilaris (PASCOE), was examined at 25°C, using six populations collected from 24 to 40^◯N in Japan (Akita, Oyama, Ayabe, Ino, Naze and Ishigaki). Under long daylength (>__= 14 h), most larvae of all populations pupated after the 4th to 6th stadium without diapause. In three populations (Ayabe, Ino and Naze), larvae reared under shorter daylength (<__= 13 h) went through about two more larval moltings and eventually entered diapause. In contrast, the southernmost (Ishigaki) and northern two (Akita and Oyama) populations did not manifest a clear photoperiodic response at the temperature tested. Geographic variation of photoperiodic response in the four southwestern populations (Ayabe, Ino, Naze and Ishigaki) was considered to exhibit a clinal change, while responses of the northern two populations (Akita and Oyama) showed discontinuous trend.