Abstract. In this paper, we show that the p-adic K-theory of a connected p-compact is the ring of invariants of the Weyl group action on the K-theory of a maximal torus. We apply this result to show that a connected finite loop space admits a maximal torus if and only if its complex K-theory is λ-isomorphic to the K-theory of some BG, where G is a compact connected Lie group.Mathematics Subject Classification (1991). Primary 55P35, 55R35, 55N15.