Recent progress in F-theory model building [1][2][3][4][5][6][7][8] 1 has shown that old successful GUTs, including the Georgi-Glashow minimal SU(5), the SO(10) model etc, are naturally realised on the world-volume of non-perturbative seven branes wrapping appropriate compact surfaces. The rather interesting fact in F-theory constructions is that they are defined on a compact elliptically fibered Calabi-Yau complex four dimensional manifold thus the exceptional groups E 6 , E 7 , E 8 , can be naturally incorporated into the theory too [1][2][3]6]. Although exceptional gauge symmetries suffer from several drawbacks when realized in the context of four-dimensional grand unified theories, in the case of F-theory models they are more promising as new possibilities arise for the symmetry breaking mechanisms and the derivation of the desired massless spectrum.Present studies on F-theory model building have been concentrated on three generation -mainly SU (5)-GUT models which fall into the following two distinct categories: those where all three families with the same Standard Model representation content are assigned to a single matter curve [11][12][13][14], and variants [15][16][17][18][19][20][21] where some or all of the quark and lepton families are assigned to different curves. Several of these constructions built up to these days have attempted to give solutions to fundamental GUT problems as is the case of doublet-triplet splitting, the rapid proton decay, the Higgs mixing term, the neutrino sector and other related issues [3,6,10,16,[21][22][23]. To analyse the phenomenological properties one should extract the relevant information from the superpotential which can be readily constructed once a particular assignment of the fermion families and Higgs on the matter curves has been chosen. Of course, dominant rôle on the estimation of such effects is played by the Yukawa couplings, thus the theory's predictive power depends on the calculability of the latter.In F-theory GUTs the trilinear Yukawa couplings are realised at the intersections of three matter curves Σ i , i = 1, 2, 3 where the zero-modes of two fermion fields and a Higgs boson reside. Along these curves the G S symmetry is enhanced G Σ i ⊃ G S ×U (1) i while the corresponding zero modes are charged under the U (1) i . To determine the most general structure of the zero-mode wavefunctions one has to solve their corresponding differential equations of motion emerging from the twisted eight-dimensional Yang-Mills action [2], (see also [14,[24][25][26]). In general, the solutions are found to exhibit the expected gaussian [24,25] profile which falls off exponentially away from the curve while their exact form is specified by a mass scale characterizing the size of the compact space and the particular U (1) i -charge of the relevant zero-mode. The Yukawa couplings of the {33}-entries of the up, down and charged lepton mass matrices are then computed in terms of the integrals of overlapping wavefunctions of the aforementioned form at the intersection point ...