We search for the production of single top quarks via flavor-changing-neutral-current couplings of a gluon to the top quark and a charm (c) or up (u) quark. We analyze 230 pb-1 of lepton+jets data from p collisions at a center of mass energy of 1.96 TeV collected by the D0 detector at the Fermilab Tevatron Collider. We observe no significant deviation from standard model predictions, and hence set upper limits on the anomalous coupling parameters / and /, where g define the strength of tcg and tug couplings, and defines the scale of new physics. The limits at 95% C.L. are /<0.15 TeV-1 and /<0.037 TeV-1.