  We present a measurement of the direct CP violating asymmetry in the decay B0
-> K+pi- using 227 x 10^6 Y(4S) -> BBbar decays collected with the BaBar
detector at the PEP-II asymmetric-energy e+e- collider at SLAC, we observe a
total signal yield of n(K-pi+) + n(K+pi-) = 1606 +- 51 decays and measure the
asymmetry [n(K-pi+)-n(K+pi-)]/[n(K-p+-)+n(K+pi-)] = -0.133+-0.030 (stat) +-
0.009 (syst). The probability of observing such an asymmetry in the absence of
direct CP violation is 1.3 x 10^-5, corresponding to 4.2 standard deviations.
