The receptor for photoperiodism in nymphs of Poecilocoris lewisi was examined using a phosphorescent paint, which absorbs light energy and emits phosphorescence in the dark. This species shows a facultative diapause in the fifth (final) nymphal instar and its induction is primarily controlled by photoperiod in the fourth instar. The incidence of diapause in the fifth instar was determined after exposing selected regions of the body surface to a longer photophase than the rest by applying a phosphorescent paint in the fourth instar. The incidence of diapause was significantly lower in insects with their compound eyes painted than in control insects at near-critical daylengths. However, painting the central part of the head had no effect. It is concluded, therefore, that the compound eyes are the principal receptor for photoperiodism in nymphs of P. lewisi. This is the first report implicating the compound eyes in the reception of photoperiod in nymphal insects.