  We present optical light curves for the period 1996-2000, of two of the
brightest known EGRET BL Lac objects: PKS 2005-489 and PKS 2155-304, the latter
also one of the few known TeV sources. For both objects, quiescent epochs of
low level of variability were followed by active periods, without any
indication of periodicity. In PKS 2005-489, several variability events with
duration of about 20 days were observed. In PKS 2155-304 fast drops and
subsequent rises in luminosity occurred in time scales of days. We proposed an
explanation in which a region moving along the relativistic jet is eclipsed by
broad line region clouds or star clusters in the host galaxy. We compare our
light curves with contemporaneous X-ray observations from All-Sky Monitor/RXTE.
Correlations between optical and X-ray activity were not found in any of the
sources at long time scales. However in PKS 2005-489 possible correlations were
observed in 1997 and 1998 at short time scales, with optical variability
preceding X-rays by 30 days in 1997 and succeeding them by about 10 days in
1998. The analysis of the SED, using the optical data presented here and
BeppoSAX contemporaneous observations obtained from the literature, shows only
small shifts in the synchrotron peak as the X-ray flux density changes.
