We consider arbitrary splits of field operators into two parts; psi = psi(+) - psi(-), and use the corresponding definition of normal ordering introduced earlier [T. S. Evans and D. A. Steer, Nucl. Phys. B 474, 481 (1996)]. In this case the normal ordered products and contractions have none of the special symmetry properties assumed in existing proofs of Wick's theorem. Despite this, we prove that Wick's theorem still holds in its usual form as long as the contraction is a c-number. Wick's theorem is thus shown to be much more general than existing derivations suggest, and we discuss possible simplifying applications of this result. (C) 1998 American Institute of Physics. [S0022-2488(98)01911-2].