  For classical groups we show the isomorphism of the Knapp-Stein $R$-group,
which describes the structure of parabolically induced representations, and the
Arthur $R$-group of the parameter associated to the inducing representation by
the local Langlands conjecture. We do this in the case of inducing from
discrete series representations. In the case of unitary groups we show this
isomorphism under a mild assumption on the parameter, which we show holds in at
least half the cases.
