  BL Lac has been observed during the outburst occurred in the 1997 summer
(Noble et al. 1997; Bloom et al. 1997) in three optical bands (BVI), with the
aim of studying the intranight colour variations on very short timescales. Due
to the high luminosity of the source we could perform quasi-simultaneous
measurements in the three bands with a resolving time of about 5 minutes using
one single telescope. We have detected several microvariability events
occurring on timescales ranging from 30 min up to 3.5 hours. The magnitude
changes had different amplitudes in the three bands being larger for shorter
wavelenghts, and the source showed a marked trend to became bluer when
brigther. There was no evidence of quasi-periodic oscillations in our
multicolour light curves.
