  We develop and study a D-brane realization of 4D N=2 super Yang-Mills theory.
It is a type IIB string theory compactified on R^6\times K3 and containing
parallel 7-branes. It can also be regarded as a subsector of Vafa's F-theory
compactified on K3\times K3 and is thus dual to the heterotic string on
K3\times T^2. We show that the one-loop prepotential in this gauge theory is
exactly equal to the interaction produced by classical closed string exchange.
A monopole configuration corresponds to an open Dirichlet 5-brane wrapping
around K3 with ends attached to two 7-branes.
