The invariant cross section as a function of transverse momentum for antide
uterons produced in 158A GeV/c per nucleon Pb + Pb central collisions has b
een measured by the NA44 experiment at CERN. This measurement, together wit
h a measurement of antiprotons, allows for the determination of the antideu
teron coalescence parameter. The extracted coalescence radius is found to a
gree with the deuteron coalescence radius and radii determined from two par
ticle correlations.