  We study possible patterns of neutrino masses and mixings in string models in
which Majorana neutrino masses are generated by a certain class of string
theory instantons recently considered in the literature. These instantons may
generate either directly the dim=5 Weinberg operator or right-handed neutrino
Majorana masses, both with a certain flavour-factorised form. A hierarchy of
neutrino masses naturally appears from the exponentially suppressed
contributions of different instantons. The flavour structure is controlled by
string amplitudes involving neutrino fields and charged instanton zero modes.
For some simple choices for these amplitudes one finds neutrino mixing patterns
consistent with experimental results. In particular, we find that a
tri-bimaximal mixing pattern is obtained for simple symmetric values of the
string correlators.
