  We present a preliminary measurement of the time-dependent CP asymmetry for
the neutral $B$-meson decay $B^0\to\phi K^0$. We use a sample of approximately
227 million $B$-meson pairs recorded at the $\Upsilon(4S)$ resonance with the
BaBar detector at the PeP-II $B$-meson Factory at SLAC. We reconstruct the $CP$
eigenstates $\phi K^0_S$ and $\phi K^0_L$ where $\phi\to K^+K^-$,
$K^0_S\to\pi^+\pi^-$, and $K^0_L$ is observed via its hadronic interactions.
The other $B$ meson in the event is tagged as either a $B^0$ or $\bar{B}^0$
from its decay products. The values of the CP-violation parameters derived from
the combined $\phi K^0$ dataset are $S_{\phi K} = +0.50\pm 0.25 ({\small
stat})^{+0.07}_{-0.04} ({\small syst})$ and $C_{\phi K} = 0.00\pm 0.23 ({\small
stat}) \pm 0.05 ({\small syst})$. In addition, we measure the CP -violating
charge asymmetry $\mathcal{A_{CP}} (B^+\to \phi K^+) = 0.054\pm 0.056 ({\small
stat}) \pm 0.012 ({\small syst})$. All results are preliminary.
