High spectral and time resolution observations have been carried out on the prototype of the beta Cephei stars. More than 650 spectra of high quality (signal-to-noise ratio above 200) were obtained on the Si III lambdalambda4553-4575 triplet lines. The radial velocity curves show a sinusoidal shape, with a 2K-amplitude varying between 29 km.s-1 and 35 km.s-1, while the gamma-velocity remains roughly constant. The line parameters, i.e. width, intensity and equivalent width show a different behaviour than the velocity variation, since they change from night to night. A frequency analysis reveals for the first time the presence of three pulsation periods in the radial velocity variation. The moment method for mode identification is then successfully applied to identify the three modes, indicating a main radial mode and two small-amplitude low-degree (probably l = 2) modes. It seems that the non-radial components are responsible for most of the line parameters variations, in which the three frequencies are also detected. In particular, the third frequency detected in the velocity variation is the first present in the full width at half maximum variation. This could be an effect of the larger associated turbulence.