  We compute critical exponents in a $Z_2$ symmetric scalar field theory in
three dimensions, using Wilson's exact renormalization group equations expanded
in powers of derivatives. A nontrivial relation between these exponents is
confirmed explicitly at the first two orders in the derivative expansion. At
leading order all our results are cutoff independent, while at next-to-leading
order they are not, and the determination of critical exponents becomes
ambiguous. We discuss the possible ways in which this scheme ambiguity might be
resolved.
