  We prove a Pohozaev type identity for non-linear eigenvalue equations of the
Dirac operator on Riemannian spin manifolds with boundary. As an application,
we obtain that the mean curvature H of a conformal immersion S^{n}-> R^{n+1}
satisfies $\int \partial_X H=0$ where X is a conformal vector field on S^{n}
and where the integration is carried out with respect to the Euclidean volume
measure of the image.<BR> This identity is analogous to the Kazdan-Warner
obstruction that appears in the problem of prescribing the scalar curvature on
S^{n} inside the standard conformal class.
