Observation of [ital D][sup 0][r arrow][ital K][sup +][pi][sup [minus]]

Using the CLEO II data sample, with an integrated luminosity of 1.8 fb[sup [minus]1] at and near the [Upsilon](4[ital S]) resonance, we have observed a signal for [ital D][sup 0][r arrow][ital K][sup +][pi][sup [minus]], which could be due to either [ital D][sup 0][ital [bar D]][sup 0] mixing or doubly Cabibbo suppressed decay, or a combination of the two. We find [ital scrB]([ital D][sup 0][r arrow][ital K][sup +][pi][sup [minus]])/[ital scrB]([ital D][sup 0][r arrow][ital K][sup [minus]][pi][sup +])=0.0077[plus minus]0 .0025(stat) [plus minus]0.0025(syst).