  This paper discusses 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 DZero detector. 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 ppbar->tb+X of 39 pb. The
upper limit for the t-channel process ppbar->tqb+X is 58 pb.
