  Given a finite group G and a G-space X, we show that a direct sum $F_G (X) =
\bigoplus_{n \geq 0}K_{G_n} (X^n) \bigotimes \C$ admits a natural graded Hopf
algebra and $\lambda$-ring structure, where $G_n$ denotes the wreath product $G
\sim S_n$. $F_G (X)$ is shown to be isomorphic to a certain supersymmetric
product in terms of $K_G(X)\bigotimes \C$ as a graded algebra. We further prove
that $F_G (X)$ is isomorphic to the Fock space of an infinite dimensional
Heisenberg (super)algebra. As one of several applications, we compute the
orbifold Euler characteristic $e(X^n, G_n)$.
