Search for the highly suppressed decays B-→K +π-π- and B-→K -K-π+

We report a search for the decays B^- → K^+π^-π^- and B^- → K^-K^-π^+, which are highly suppressed in the standard model. Using a sample of (467±5)×10^6 B B pairs collected with the BABAR detector, we do not see any evidence of these decays and determine 90% confidence level upper limits of Β (B^- → K^+π^-π^-)<9.5×10^(-7) and Β(B^- → K^-K^-π^+)<1.6×10^(-7) on the corresponding branching fractions, including systematic uncertainties.