  We investigate the rotational velocities of early-type stars in the Sco OB2
association. We measure \vsini\ for 156 established and probable members of the
association. The measurements are performed with three different techniques,
which are in increasing order of expected \vsini: 1) converting the widths of
spectral lines directly to \vsini, 2) comparing artificially broadened spectra
of low \vsini\ stars to the target spectrum, 3) comparing the \ionI{He}4026\
line profile to theoretical models. The sample is extended with literature data
for 47 established members of Sco OB2. Analysis of the \vsini\ distributions
shows that there are no significant differences between the subgroups of Sco
OB2. We find that members of the binary population of Sco OB2 on the whole
rotate more slowly than the single stars. In addition, we find that the B7--B9
single star members rotate significantly faster than their B0--B6 counterparts.
We test various hypotheses for the distribution of \vsini\ in the association.
The results show that we cannot clearly exclude any form of random distribution
of the direction and/or magnitude of the intrinsic rotational velocity vector.
We also investigate the effects of rotation on colours in the Walraven
photometric system. We show that positions of B7--B9 single dwarfs above the
main sequence are a consequence of rotation. This establishes the influence of
rotation on the Walraven colours, due primarily to surface gravity effects.

Astrophysics