Context :

Aims : We perform a seismic study of the young massive \beta Cephei star HD 203664 with the goal to constrain its interior structure .

Methods : Our study is based on a time series of 328 new Geneva 7-colour photometric data of the star spread over 496.8 days .

Results : The data confirm the frequency of the dominant mode of the star which we refine to f _ { 1 } = 6.02885 c d ^ { -1 } .
The mode has a large amplitude of 37 mmag in V and is unambiguously identified as a dipole mode ( \ell = 2 ) from its amplitude ratios and non-adiabatic computations .
Besides f _ { 1 } , we discover two additional new frequencies in the star with amplitudes above 4 \sigma : f _ { 2 } = 6.82902 c d ^ { -1 } and f _ { 3 } = 4.81543 c d ^ { -1 } or one of their daily aliases .
The amplitudes of these two modes are only between 3 and 4 mmag which explains why they were not detected before .
Their amplitude ratios are too uncertain for mode identification .

Conclusions : We show that the observed oscillation spectrum of HD 203664 is compatible with standard stellar models but that we have insufficient information for asteroseismic inferences .
Among the large-amplitude \beta Cephei stars , HD 203664 stands out as the only one rotating at a significant fraction of its critical rotation velocity ( \sim 40 \% ) .