The collection efficiency of a commercial electrostatic aerosol sampler has been measured at flow rates of 4 and 6 l. min −1 with monodisperse 0.500, 1.099, 1.947 μ dia. aerosols. Efficiencies were in the range of 80–90 per cent and sample collection was only slightly biased with respect to particle size. Aerosol deposition was uniform on glass muscope slides but not on Millipore filters. Aerosol deposition on the coated side of electron muscope grids was equal to deposition on the glass slides but was one-half less on the uncoated side. This discrepancy was tentatively attributed to preferential deposition on the grid wires.