An event consistent with the signature expected for the rare kaon decay K+ ! �+�¯ � has been observed. In the pion momentum region examined, 211 < P < 230 MeV/c, the backgrounds are estimated to contribute 0.08 ± 0.03 events. If the event is due to K+ ! �+�¯�, the branching ratio is 4.2+9.7 −3.5× 10−10.
