The activity recording system used to monitor circadian periodicity in pink bollworm moths, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), is presented. The actograph provides a permanent record while eliminating the use of light and direct observation for studying nocturnal periodicity. A graphic recorder, preamplifier, and power supply register the output of a vibration-sensitive transducer mounted on an isolated metal cage. Vibrations produced by movements of moths in the cage indicate the overall levels of activity which are associated with specific behavioral events. Response to light, foraging behavior, mating, and relative inactivity are indicated by characteristic patterns. Rhythms produced by 20 moths during a continuous 14-day trial include entrainment, free-run in dark, and subsequent reentrainment. Circadian periodicity was dependent on photoperiodic induction. This system is highly adaptable and may be used with a variety of arthropods.