  We report the first observation of the charmless hyperonic B decay, B0 -> p
Lambdabar pi-, using a 78 fb^-1 data sample recorded on the Upsilon(4S)
resonance with the Belle detector at KEKB. The measured branching fraction is
B(B0 -> p Lambdabar pi-) = (3.97 ^{+1.00}_{-0.80} +- 0.56) * 10^-6. Searches
for B0 -> p Lambdabar K- and p Sigmabar0 pi- yield no significant signals and
we set 90% confidence level upper limits of B(B0 -> p Lambdabar K-) < 8.2 *
10^-7 and B(B0 -> p Sigmabar0 pi-) < 3.8 * 10^-6.
