Social stimulation is not usually considered to be a circadian synchronizer of activity in insects. We investigated whether it could have any effect on synchronization and resetting capacity in insects with cauterized eyes. Direct continuous observations were carried out in groups ofPeriplaneta americanacomposed of two operated animals and eight controls. When night fell at the same time as before surgery, sexual and locomotor activity of operated animals remained synchronized with the activity of control animals. But when nightfall was postponed by 6 h, operated animals were unable to lengthen the activity peak, as long as the activity threshold stayed at its minimum level. Social stimulation resulted in modifications of and interactions with the temporal activity distribution within the threshold limits, but was unable to reset the circadian clock; so it may be questioned whether it should be regarded as a circadian synchronizer in a strict sense. Circadian rhythm of food intake disappeared in operated animals; this unstable character which has already been mentioned by other authors, even in intact animals, is discussed.