  Using e^+e^- -> D_s^*+ D_s^- data collected near the peak D_s production
energy, E_cm=4170 MeV, with the CLEO-c detector, we present the first
observation of the decay D_s^+ -> proton anti-neutron. We measure a branching
fraction B(D_s^+ -> p anti-n = (1.30 +- 0.36 +0.12 -0.16) x 10^-3. This is the
first observation of a charmed meson decaying into a baryon-antibaryon final
state.
