The Shower-Maximum Detector (SMD) of the CDF Plug Upgrade EM
calorimeter has been tested at the 1996-97 test beam.  Calibration of the SMD
has been performed using both beam positron response and wire-source data,
with both calibrations agreeing to within 5%.  The e^+ position resolution was
studied by fitting the shower profile to find its center and comparing with the
projected track from the drift chambers.  A position resolution of (38/E +
0.4) mm for positrons in the energy range 10-180 GeV has been obtained.