  We compute the leading corrections to the differential cross section for
top-pair production via gluon fusion due to dimension-six operators at leading
order in QCD. The Standard Model fields are assumed to couple only weakly to
the hypothetical new sector. A systematic approach then suggests treating
single insertions of the operator class containing gluon field strength tensors
on the same footing as expli\-citly loop suppressed contributions from
four-fermion operators. This is in particular the case for the chromomagnetic
operator $Q_{(uG)}$ and the purely bosonic operators $Q_{(G)}$ and $Q_{(\varphi
G)}$. All leading order dimension-six contributions are consequently suppressed
with a loop factor $1/16\pi^2$.
