The body cavities of two male fireflies were put in contact by means of a drop of insect physiological saline. The eyes of firefly A were stimulated with a beam of light, whereas those of firefly B were covered with an opaque paste. Photic stimulation of the eyes of firefly A produced a blockade of its flashing as well as a reduction in intensity and frequency of flashing of firefly B. Control experiments ruled out the possibility of direct photic stimulation of the second firefly. The inhibitory effect on flashing of firefly B was not affected by ablation of the lantern but disappeared after gonadectomy of firefly A. In a single firefly inhibition by illumination or by low rate electrical stimulation of the ventral cord of the flashes elicited by lantern stimulation was no longer present after bilateral gonadectomy. It is concluded that a flash inhibiting substance is released from the male gonads upon photic stimulation and that it exerts its action at the level of the photogenic organ.