S-duality and the dyon spectrum in N=2 super Yang-Mills theory

We study the dyon spectrum in N=2 super Yang-Mills theory with gauge group SU(2) coupled to Nf matter multiplets in the fundamental representation. For magnetic charge one and two we determine the spectrum explicitly and show that it is in agreement with the duality predictions of Seiberg and Witten. We briefly discuss the extension to higher charge monopoles for the self-dual Nf=4 case and argue that the conjectured spectrum of dyons predicts the existence of certain harmonic spinors on the moduli space of higher charge monopoles. (orig.)