Abstract:We present a spectroscopic study of the beta Cephei star beta Canis Majoris. Hundreds of high-resolution spectra were gathered during 4.5 years and the Si III line profiles centered on 4560 A were subjected to a detailed line profile analysis. We searched for oscillation frequencies in different line diagnostics, compared them with those known from photometric studies and concluded the presence of three frequencies in our dataset: f_1 = 3.97933 c/d, f_2 = 3.99959 c/d and f_3= 4.1832 c/d, which were already found earlier from photometric data. By means of the moment method and from amplitude and phase variations across the profiles we could identify two of the modes as (l_1,m_1)=(2,2) and (l_2,m_2)=(0,0$. We present our first results of modelling based on the oscillation frequencies and place constraints on the mass, age and core overshooting of this massive evolved beta Cephei star.