  We used Johnson UBV photometric CCD observations to identify pulsating and
other variable stars in the young open cluster NGC 6231. The multi-color
information was used to classify pulsating variables, perform frequency
analysis, and - where possible - to compare observed to theoretical amplitude
ratios for mode identification. The data reduction was performed with standard
IRAF tools. Differential light curves have been obtained by identifying a set
of suitable comparison stars and the frequency analysis was then conducted on
the basis of Fourier methods. Our classification of pulsating stars was based
on the time scales and amplitudes of the variability with respect to the
different filters and stellar parameters as calculated from published
Str\"omgren and Geneva photometry. We identified 32 variable stars in the field
of the cluster out of which 21 are confirmed members and twelve are newly
detected variable stars. Ten stars were classified as Slowly Pulsating B (SPB)
stars in NGC 6231 out of which seven are new discoveries. We also analyzed six
previously reported {\beta} Cephei variables in more detail. One of them may be
a hybrid {\beta} Cephei/SPB pulsator. In addition, we investigated five more
previously suspected pulsators of this group which we cannot convincingly
confirm. The remaining eleven variable stars are either not members of NGC 6231
or the membership status is questionable. Among them are three previously known
{\delta} Scuti stars, two newly detected pulsators of this class, one new and
two already known eclipsing binaries, one new SPB variable, one possible
Pre-Main-Sequence (PMS) pulsator and another new variable star for which we
cannot present a classification. With more than 20 main sequence pulsators of
spectral type B, NGC 6231 becomes the open cluster with the largest population
of such pulsating stars known.
