The particle counts of the Acoustical Particle Sizing Device (APSD) developed by Coover in 1978 was compared with that of the Climet 208 Optical Particle Analyzer (OPA) and the Gardner Condensation Nuclei Counter (CNC). The OPA was used as the reference machine in counting lycopodium spores using three different set-ups, as well as polystyrene latex spheres (PSL) of different diameters and concentration levels. The CNC was used as the standard in counting particles from cigarette smoke, magnesium strip fumes and the reaction of sodium hydroxide (NaOH) pellets and nitric acid. The mean difference in the number of particles per liter counted by both machines was computed for every trial, and the statistical significance was tested; the correlation coefficient was also determined. Results indicate the inability of the APSD to count particles with diameters below 5 μm.