The specific heat of a single crystal sample of YBa2Cu3O7-delta has be
en measured with magnetic fields up to 8 T applied parallel to the c a
xis of the crystal. The results provide strong evidence for the existe
nce of a critical regime within which there is scaling behavior charac
teristic of the three-dimensional X-Y model with critical exponents co
nsistent with those observed in superfluid He-4.