The sampling mechanism of a thin-walled, cylindrical aerosol sampling probe which faces directly away from the wind (180 degrees orientation) is numerically investigated. The turbulent fluid flow is predicted by employing the control volume, finite-difference method and the k-epsilon turbulence model. The particle trajectories are calculated by integrating the particle equations of motion and thus the aspiration efficiency of the sampler is determined. The numerical results have been obtained for two very long thin-walled cylindrical samplers of diameters D = 2 and 5 cm which have zero thickness and the operating conditions as used in the experimental investigations of Vincent et al. (1986, J. Aerosol Sci. 17, 211-224) have been considered. The dependence of the characteristics of the fluid flow and the aspiration efficiency on the freestream air speed, the diameter of the sampler and the ratio of the freestream speed to the sampling speed, have been thoroughly investigated. It is concluded that the numerical results for the aspiration efficiency are in reasonable agreement with all the existing experimental data for thin-walled samplers which are placed at 180 degrees to the direction of the wind.