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We present a search for electroweak production of single top quarks in the electron+jets and muon+jets decay channels. The measurements use ≈90 pb-1 of data from Run 1 of the Fermilab Tevatron collider, collected at 1.8 TeV with the D Ø detector between 1992 and 1995. We use events that include a tagging muon, implying the presence of a b jet, to set an upper limit at the 95% confidence level on the cross section for the s-channel process pp̅→tb+X of 39 pb. The upper limit for the t-channel processpp̅→tqb+X is 58 pb.