  In 0.9 fb$^{-1}$ of $p \bar p$ collisions, D0 has observed an excess of
events with an isolated lepton, missing transve rse momentum, and two to four
jets. This excess is consistent with single top quark production. We examine
these data to study the Lorentz structure of the $Wtb$ coupling. The standard
model predicts a left-handed vector coupling at the $Wt b$ vertex. The most
general lowest dimension, CP-conserving Lagrangian admits right-handed vector
and left- or right-h anded tensor couplings as well. We find that the data
prefer the left-handed vector coupling and set upper limits on the anomalous
couplings. These are the first direct constraints on a general $Wtb$
interaction and the first direct limits on left- and right-handed tensor
couplings.
