  We present a stringy mechanism to generate Dirac neutrino masses by
D-instantons in an experimentally relevant mass scale without fine-tuning.
Within Type IIA string theory with intersecting D6-branes, we spell out
specific conditions for the emergence of such couplings and provide a class of
supersymmetric local SU(5) Grand Unified models, based on the Z_2 x Z'_2
orientifold compactification, where perturbatively absent Dirac neutrino masses
can be generated by D2-brane instantons in the experimentally observed mass
regime, while Majorana masses remain absent, thus providing an intriguing
mechanism for the origin of small neutrino masses due to non-perturbative
stringy effects.
