  I present a calculation of threshold soft-gluon corrections to single top
quark production in ppbar collisions via all partonic processes in the t and s
channels and via associated top quark and W boson production. The soft-gluon
corrections are formally resummed to all orders, and finite-order expansions of
the resummed cross section are calculated through
next-to-next-to-next-to-leading order (NNNLO) at next-to-leading logarithmic
(NLL) accuracy. Numerical results for single top quark production at the
Tevatron are presented, including the dependence of the cross sections on the
top quark mass and on the factorization and renormalization scales. The
threshold corrections in the t channel are small while in the s channel they
are large and dominant. Associated tW production remains relatively minor due
to the small leading-order cross section even though the K factors are large.
