  We present a search for electroweak production of single top quarks in the
s-channel and t-channel using neural networks for signal-background separation.
We have analyzed 230 pb$^{-1}$ of data collected with the D0 detector at the
Fermilab Tevatron Collider at a center-of-mass energy of 1.96 TeV and find no
evidence for a single top quark signal. The resulting 95% confidence level
upper limits on the single top quark production cross sections are 6.4 pb in
the s-channel and 5.0 pb in the t-channel.
