Physical quark-number charges of dyons are determined, via a formula which generalizes that of Witten for the electric charge, in N = 2 supersymmetric theories with SU(2) × U(1)^N_f gauge group. The quark numbers of the massless monopole at a nondegenerate singularity of the space of vacua turn out to vanish in all cases. A puzzle related to CP invariant cases is solved. Generalization of our results to SU(N_c) × U(1)^N_f gauge theories is straightforward.