  We present a preliminary measurement of the time-dependent CP-violating
asymmetry in the neutral B decay B0 --> phi K0S, with phi --> K+K- and K0S -->
pi+pi-. The measurement uses a data sample of about 87 million Y(4S) --> B Bbar
decays collected between 1999 and 2002 with the BaBar detector at the PEP-II
asymmetric-energy B Factory at SLAC. In this sample we study events in which
the CP final state is fully reconstructed and the flavor of the other neutral B
meson is determined from its decay products. The amplitude of the CP-violating
asymmetry sin2beta is derived from the decay-time distributions. We measure
sin2beta = -0.19 +0.52-0.50 (stat) +/- 0.09 (syst) (preliminary).
