We present a search for electroweak production of single top quarks in
the s-channel and t-channel modes. We have analyzed 230 pbÐ1 of data
collected with the D0 detector at the Fermilab Tevatron collider at a
center-of-mass energy of 1.96 TeV. Three separate analysis methods are used: neural
networks, decision trees, and a cut-based analysis. No evidence for a single
top signal is found. We set 95% confidence level Bayesian upper limits on the
production cross sections using binned likelihood fits to the neural network and 
decision tree output distributions and using the total numbers ofevents 
in the cut-based analysis. The limits from the neural networks
(decision trees, cut-based) analysis are 6.4 pb (8.3 pb, 10.6 pb) in the s-channel and 5.0 pb  (8.1 pb, 11.3 pb) in the t-channel.