  We evaluate the angular bispectrum of the CMB temperature anisotropy at large
angular scale due to a stochastic background of primordial magnetic fields. The
shape of non-Gaussianity depends on the spectral index of the magnetic field
power spectrum and is peaked in the squeezed configuration for a
scale-invariant magnetic spectrum. By using the large angular part of the
bispectrum generated by magnetic fields, the present bounds on non-Gaussianity
set a limit on the amplitude of the primordial magnetic field of the order of
10 nGauss for the scale-invariant case and 20 nGauss for the other spectral
indexes.
