  We study the scaling behaviour of the pseudo-critical couplings for the
chiral phase transition in two-flavour QCD. We show that all existing results
from lattice simulations on lattices with temporal extent $N_\tau = 4$, 6 and 8
can be mapped onto a universal scaling curve. The relevant combination of
critical exponents, $\beta\delta$, is consistent with the scaling behaviour
expected for a second order phase transition with $O(4)$ exponents. At present,
scaling according to the $O(2)$ symmetry group can, however, not be ruled out.
