Two different atomic species-sodium and cesium-have been resonantly excited in a cell by a cw dye laser and a cesium lamp, respectively. The density of the excited Na(3P) atoms in the cell is measured directly using the absorption-equivalent-width technique on the lines of the diffuse series. To isolate the heteronuclear contribution in the fluorescence spectrum, a modulation detection technique has been adopted. The energy-pooling collisions between Na(3P) and Cs(6P) have been observed and cross sections have been measured.