  The comultiplication formula for fusion products of untwisted representations
of the chiral algebra is generalised to include arbitrary twisted
representations. We show that the formulae define a tensor product with
suitable properties, and determine the analogue of Zhu's algebra for arbitrary
twisted representations.
  As an example we study the fusion of representations of the Ramond sector of
the N=1 and N=2 superconformal algebra. In the latter case, certain subtleties
arise which we describe in detail.
