  We present results of a photometric CCD study of the incidence of
microvariability in the optical emission of a sample of 20 blazars detected at
gamma-ray energies by the EGRET instrument of the Compton Gamma-Ray
Observatory. We have observed strong outbursts in some sources, but many others
displayed no significant variability on timescales of hours. The typical
minimum timescale results to be of ~ several hours, not tens of minutes as
claimed by some authors. The duty cycle for optical intranight microvariations
of gamma-ray blazars, as estimated from our observations, seems to be ~50%,
lower than what is usually assumed. For night-to-night variations, instead, the
duty cycle approaches to what is observed in radio-selected BL Lacs and
flat-spectrum radio quasars (i.e. ~70%).
