  The purpose of this paper is to generalize Zhu's theorem about characters of
modules over a vertex operator algebra graded by integer conformal weights, to
the setting of a vertex operator superalgebra graded by rational conformal
weights. To recover SL_2(Z)-invariance of the characters it turns out to be
necessary to consider twisted modules alongside ordinary ones. It also turns
out to be necessary, in describing the space of conformal blocks in the
supersymmetric case, to include certain `odd traces' on modules alongside
traces and supertraces. We prove that the set of supertrace functions, thus
supplemented, spans a finite dimensional SL_2(Z)-invariant space. We close the
paper with several examples.
