  We present a search for electroweak production of single top quarks in ~90
pb^-1 of data collected with the DZero detector at the Fermilab Tevatron
collider. Using arrays of neural networks to separate signals from backgrounds,
we set upper limits on the cross sections of 17 pb for the s-channel process
ppbar->tb+X, and 22 pb for the t-channel process ppbar->tqb+X, both at the 95%
confidence level.
