1. Microphotographs were obtained of a firefly (Photuris sp.) light organ on cinema film at 64 frames per second with the aid of an image intensifier. 2. Analysis of the film indicates that the two light organs usually flash simultaneously, but cases of one organ leading by 32-48 msec are seen. 3. Asynchrony is also observed within the light organ during a flash. Submaximal flashes are comprised of small spots of light ("microsources") which turn on at various times throughout the flash. Their durations average 139 ± 3 (S.E.) msec (range 31-360 msec) compared with 300-425 msec for an organ flash. Microsources are of fairly uniform size and shape having an average diameter of 21.6 ± 0.3 µ. (S.E.) and a range of 11-36 µ. Microsources of this size are also seen as time functional unit of uncoordinated emission displays known as scintillation. It is suggested that the microsource is the smallest physiological unit of function in the firefly light organ. 4. Comparisons of glowing organs with histological sections demonstrate that the center portions of the "rosettes" are dark, and the centrifugal elements of the rosettes (the photocytes) emit light. 5. Microsources (21.6 µ in diameters) are somewhat smaller than the measured length of photocytes (34 µ) but larger than the width (8-10 µ). Thus the functional unit is identified as the center portions of 2 or 3 adjacent photocytes. 6. The whole organ flash is the integration of briefer staggered responses of a population of these single units which have activities of variable duration and intensities.