The deposition of ultrafine (d less than or equal to 200 nm) particles on airway surfaces is an important determinant of the radiation dose that results from inhalation of radon progeny. Diffusion is the dominant deposition mechanism for radon progeny since most of the alpha particle activity is on ultrafine particles. Freshly formed Po-218 is rapidly neutralized but, there remains some charged fraction of each short-lived decay product. Theoretical predictions suggest that a measurable increase in airway deposition may result from particle charge. We have measured and compared the deposition (eta) of monodisperse singly charged, and charge neutralized, particles with diameters from 15 to 95 nm in simple tracheal models. Differences in deposition were detectable for particles < 30 nm in diameter in 10, 23 and 30 cm long tubes, and for particles up to 95 nm for the longest (30 cm) tube tested. Variations in the magnitude of electrostatic deposition with particle and flow parameters is consistent with theoretical predictions.