Abstract. Let G be a semisimple Lie group with Iwasawa decomposition G = KAN . Let X = G/K be the associated symmetric space and assume that X is of rank one. Let M be the centraliser of A in K and consider an orthonormal basis {Y δ,j :, λ ∈ C, be the Helgason Fourier transform. Let h t be the heat kernel associated to the Laplace-Beltrami operator and let Q δ (iλ + ) be the Kostant polynomials. We establish the following version of Hardy's theorem for the Helgason Fourier transform: Let f be a function on G/K which satisfies |f (ka r )| ≤ Ch t (r). Further assume that for every δ and j the functions2 for λ ∈ R. Then f is a constant multiple of the heat kernel h t .