ABSTRACT. From a Japanese population of Chymomyza costata which has been known to have a photoperiodic larval diapause, we selected a mutant strain which did not respond to photoperiod. However, about 70% of the individuals of this strain entered diapause at 11oC irrespective of photoperiod, and about the same percentage of those of the photoperiod-sensitive strain also did so in continuous illumination at 11oC. This indicates that low temperature induces diapause independently of photoperiod. On the other hand, a temperature drop from 18 or 25oC to 15oC and chilling at 4oC did not induce diapause.