The role of the circadian clock gene Clock in the circadian rhythm and the photoperiodic regulation of reproductive diapause in the bean bug Riptortus pedestris (Fabricius) (Hemiptera: Alydidae) is investigated. Clock RNA interference (RNAi) disrupts the circadian rhythm in an alternating deposition of polarized and nonpolarized cuticle layers in the endocuticle, and produces only a polarized layer. This indicates that Clock is a core component of the circadian clock and that it acts as a positive element in activating the transcription of downstream genes. In addition, Clock RNAi suppresses ovarian development irrespective of day-length conditions, which indicates that Clock is involved in the photoperiodic response. The observed phenotypes in Clock RNAi insects, the production of a single polarized layer in the endocuticle and an arrested ovarian development irrespective of photoperiod are the same as those induced by RNAi of cycle, a positive element in the circadian clock. However, the phenotypes induced by RNAi of negative elements period and mammalian-type cryptochrome differ from those induced by the positive elements. Considering the current data together with previously published findings, it is concluded that the circadian clock, comprising the circadian clock genes, is involved in the photoperiodic response of R. pedestris.