We consider the electromagnetic inverse scattering problem of determining t
he support of an orthotropic or anisotropic anomaly in a homogeneous backgr
ound medium from a knowledge of the electric far-field pattern correspondin
g to the scattering of electromagnetic plane waves. Our aim is to derive a
linear integral equation of the first kind from which the support of the an
omaly can be found by examining the behaviour of the (regularized) solution
. The analysis is based on a study of an interior transmission problem for
Maxwell's equations.