A photo-electronic fog counter has been developed on the principle of counting flashes of light scattered from individual fog droplets with a photomultiplier. Since a photomultiplier appears to act very rapidly and reponse in a time of the order of 10-8 sec. is possible, the maximum counting rate is limited only by the sealer which is associated with the counter. If a Berkeley Sealer (Model 1000B) is used, this counter is capable of counting fog droplets at a rate of 300,000/min. with very small error and the chance of coincidence countings is therefore small. This counter will prove useful for even very dense fogs or any other kind of aerosols. The optical theory of scattering which may lead to the determination of particle size from size of electric pulse has been introduced.Preliminary experiments show the applicability of this counter for droplet size measurement. There seems to be of no doubt that this counter could register the smallest droplets that occur in natural fogs (i.e., of the order of r≈1 micron) and a better sensitivity can be achieved by reducing stray-light noise level and improving signal to noise ratio. Possibilities of further improvements of the present instrument both as a fog counter and as a droplet size indicator have been discussed.