  We study the non-perturbative, instanton-corrected effective action of the
N=2 SU(2) x SU(2) supersymmetric Yang-Mills theory with a massless
hypermultiplet in the bifundamental representation. Starting from the
appropriate hyperelliptic curve, we determine the periods and the exact
holomorphic prepotential in a certain weak coupling expansion. We discuss the
dependence of the solution on the parameter q=L2^2/L1^2 and several other
interesting properties.
