  We study the topological susceptibility, chi, in two flavour lattice QCD. We
find clear evidence for the expected suppression of chi at small quark mass.
The estimate of the pion decay constant, f_pi = 105 +/-5 +18/-10 MeV, is
consistent with the experimental value of approximately 93 MeV. We compare chi
to the large-N_c prediction and find consistency over a large range of quark
masses.
