We present a search for electroweak production of single top quarks in the electron+jets and muon+jets decay channels. The measurements use ≈90pb−1 of data from Run 1 of the Fermilab Tevatron collider, collected at 1.8 TeV with the DØ 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 p¯p→tb+X of 39 pb. The upper limit for the t-channel process p¯p→tqb+X is 58 pb.Received 16 August 2000DOI:https://doi.org/10.1103/PhysRevD.63.031101©2000 American Physical Society