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Abstract. Locally smooth S'-actions on simply connected 4-manifolds are studied in terms of their weighted orbit spaces. An equivariant classification theorem is proved, and the weighted orbit space is used to compute the quadratic form of a given simply connected 4-manifold with S '-action. This is used to show that a simply connected 4-manifold which admits a locally smooth 5'-action must be homotopy equivalent to a connected sum of copies of S4, CP2, -CP2, and S2 x S2.