Alkali resonance lines are long known to be affected by radiation trapping in dense vapour media. In this report we demonstrate effects of radiation trapping in a collimated supersonic beam of Na atoms. The atomic beam was crossed by a laser at right angles, while laser excitation spectrum of sodium D2-line was measured at the angles of 45° and 90° with respect to the atomic beam axis. The following unexpected features were observed in the excitation spectrum. Firstly, itwas found that the spectral lines are broader or narrower than the expected Doppler width for observation at 45° and 90°, respectively. Secondly, the spectral components corresponding to excitation of different hyperfine levels exhibited intensity ratios different from those expected from the theoretical line strengths. Both findings are explained by a combination of effects of optical pumping and radiation trapping (RT). At low laser intensities, when nonlinear optical effects can be neglected, RT can strongly affect lineshapes of the HFS components. Comparison of the excitation spectrum measured at 45° with the calculation for different atom densities in the beam is shown in Fig.1. The simulation is based on the approach described in , and it takes into account anisotropy of the velocity distribution of atoms in the beam. The best agreement between the theory and the experiment was observed for n 3S=3.6×1010 cm-3, which is close to the density calculated from the saturated vapor pressure and geometry of the beam. Sensitivity of RT to the density of atoms in the beam can be exploited as a tool to measure atom densities in atomic beams.