Abstract
Four-dimensional N=1 supersymmetric Spin(N) gauge theories with matter in the vector and spinor representations are considered. Dual descriptions are known for some of these theories. It is noted that when masses are given to all fields in the spinor representation, the dual gauge group G breaks to a group H such that π2(G/H) = Z2. The quantum numbers of the associated Z2 monopole and those of the massive spinors are shown to agree, suggesting that the monopole is the image of the massive spinors under duality. It follows that electric sources in the spinor representation, needed as test charges to determine the phase of an SO(N) gauge theory, can be introduced asZ2-valued magnetic sources in the dual nonabelian gauge theory. This fact is used to study the phases of SO(N) gauge theories with matter in the vector representation.