Apparatus for the automatic recording of the concentration of submicrometre particles in air is described. The system employs the 1946 pattern of the Pollak photoelectric condensation nucleus counter, and is designed so that this instrument is operated in accordance with original calibration data. The operational cycle is controlled by a mechanical timer, and data are recorded by digital equipment in a form suitable for automatic computation of nucleus concentration. Intercalibration data for two similar counters compare favourably with earlier published work, and the system has proved accurate and reliable for over 1000 hours sampling in atmospheric experiments.