  We discuss the dynamics of three dimensional Maxwell theory coupled to a
level $k$ Chern-Simons term. Motivated by S-duality in string theory we argue
that the theory admits an S-dual description. The S-dual theory contains a
non-gauge 1-form field, previously proposed by Deser and Jackiw and a level $k$
$U(1)$ Chern-Simons term, $Z_{\rm MCS}=Z_{\rm DJ}Z_{\rm CS}$. The couplings to
external electric and magnetic currents and their string theory realisations
are also discussed.
