  We observe a signal for the doubly charmed baryon Xi_cc^+ in the charged
decay mode Xi_cc^+ --> Lambda_c^+ K- pi+ in data from SELEX, the charm
hadro-production experiment at Fermilab. We observe an excess of 15.9 events
over an expected background of 6.1 +/- 0.5 events, a statistical significance
of 6.3sigma. The observed mass of this state is (3519 +/- 1) MeV/c^2. The
Gaussian mass width of this state is 3MeV/c^2, consistent with resolution; its
lifetime is less than 33fsec at 90% confidence.
