  We report constraints on antineutrino oscillation parameters that were
obtained by using the two MINOS detectors to measure the 7% muon antineutrino
component of the NuMI neutrino beam. In the Far Detector, we select 130 events
in the charged-current muon antineutrino sample, compared to a prediction of
136.4 +/- 11.7(stat) ^{+10.2}_{-8.9}(syst) events under the assumption
|dm2bar|=2.32x10^-3 eV^2, snthetabar=1.0. Assuming no oscillations occur at the
Near Detector baseline, a fit to the two-flavor oscillation approximation
constrains |dm2bar|<3.37x10^-3 eV^2 at the 90% confidence level with
snthetabar=1.0.
