  Using the ASAS data, we determine the pulsational frequencies of the
high-amplitude $\delta$ Sct star BP Pegasi. The analysis revealed only the two
known, independent frequencies. On the basis of multicolour Str\"omgren
photometry, we independently find that both frequencies can only be associated
with radial modes which, according to the frequency ratio, are fundamental and
first overtone modes. The models fitting the two frequencies depend strongly on
the opacity data. For low values of the mixing length parameter $\alpha_{\rm
MLT}\approx 0.5$, only the OPAL seismic models in the post-main sequence phase
of evolution are caught within the observed error box. Seismic OP and OPLIB
models can only reach the error box if we increase $\alpha_{\rm MLT}$ to at
least 2.0. Then, including the non-adiabatic parameter $f$ into our seismic
modelling, we constrain various parameters, employing Monte Carlo-based
Bayesian analysis.
