  We present an improved measurement of the standard model CP violation
parameter sin2phi_1 (also known as sin2beta) based on a sample of 85 times 10^6
B Bbar pairs collected at the Upsilon(4S) resonance with the Belle detector at
the KEKB asymmetric-energy e+e- collider. One neutral B meson is reconstructed
in a J/psi K_S, psi(2S) K_S, chi_{c1} K_S, eta_c K_S, J/psi K^{*0}, or J/psi
K_L CP-eigenstate decay channel and the flavor of accompanying B meson is
identified from itsdecay products. From the asymmetry in the distribution of
the time intervals between the two B meson decay points, we obtain sin2phi_1 =
0.719 +/- 0.074(stat) +/- 0.035(syst). We also report measurements of CP
violation parameters for the related B^0 -> J/psi pi^0 decay mode and the
penguin-dominated processes B^0 -> eta' K_S, phi K_S and K^+K^- K_S.
