We examined the effects of alveolar duct structure on particle deposition in the pulmonary acinus. The low Reynolds number velocity field of carrier gas in a geometric model of the alveolated duct was solved numerically. Particle trajectories were computed from the Langevin equation. Conditional probabilities of the trajectories were calculated with an eigenfunction expansion technique in the absence of gravity. For submicron particles, Brownian motion dominated the process; the deposition rate dramatically decreased with boundary layer growth. For fine particles, fully developed boundary layer profiles determined the deposition over most of the acinar length. The assumption of a uniform radial profile results in a substantial overestimation of the local deposition rate. The deposition rate in an alveolated duct was always smaller than that in an equivalent straight tube of the same volume. Within the alveolus the deposition pattern was markedly nonuniform, with higher deposition near the alveolar entrance ring; this finding is consistent with experimental observations in animals (e.g., see Zeltner et al. J. Appl. Physiol. 70: 1137-1145, 1991). We conclude that the structure of the alveolar duct has an important influence on aerosol particle deposition in the lung acinus.