The authors report high-precision spectrophotometric observations of four planetary transits of HD 209458, focusing on the sodium resonance doublet at 589.3 nm. They find that the photometric dimming during transit in a bandpass centered on the sodium feature is deeper by $(2.32 \pm 0.57) \times 10^{-4}$ compared to simultaneous observations in adjacent bands. This additional dimming is interpreted as absorption from sodium in the planetary atmosphere, consistent with recent theoretical predictions. However, the observed sodium absorption is less than predicted by models with a cloudless planetary atmosphere and a solar abundance of sodium in atomic form. Several possibilities are discussed to account for this reduced amplitude, including the reaction of atomic sodium into molecular gases or condensates, photoionization by stellar flux, a low primordial abundance of sodium, or the presence of high-altitude clouds. The study highlights the potential for using existing instruments to investigate the atmospheres of extrasolar planets and suggests that future observations over a broader wavelength range could help distinguish between these models.The authors report high-precision spectrophotometric observations of four planetary transits of HD 209458, focusing on the sodium resonance doublet at 589.3 nm. They find that the photometric dimming during transit in a bandpass centered on the sodium feature is deeper by $(2.32 \pm 0.57) \times 10^{-4}$ compared to simultaneous observations in adjacent bands. This additional dimming is interpreted as absorption from sodium in the planetary atmosphere, consistent with recent theoretical predictions. However, the observed sodium absorption is less than predicted by models with a cloudless planetary atmosphere and a solar abundance of sodium in atomic form. Several possibilities are discussed to account for this reduced amplitude, including the reaction of atomic sodium into molecular gases or condensates, photoionization by stellar flux, a low primordial abundance of sodium, or the presence of high-altitude clouds. The study highlights the potential for using existing instruments to investigate the atmospheres of extrasolar planets and suggests that future observations over a broader wavelength range could help distinguish between these models.