We report the observation of the decay B-→Ds(*)+K-l-ν̅ l based on 342 fb-1 of data collected at the Υ(4S) resonance with the BABAR detector at the PEP-II e+e- storage rings at SLAC. A simultaneous fit to three Ds+ decay chains is performed to extract the signal yield from measurements of the squared missing mass in the B meson decay. We observe the decay B-→Ds(*)+K-l-ν̅ l with a significance greater than 5 standard deviations (including systematic uncertainties) and measure its branching fraction to be B(B-→Ds(*)+K-l-ν̅ l)=[6.13-1.03+1.04(stat)±0.43(syst)±0.51(B(Ds))]×10-4, where the last error reflects the limited knowledge of the Ds branching fractions.
