Let M be an oriented, connected, smooth, Riemannian manifold of dimension n > 1, and let div (κ grad ϕ) = ς ∂ϕ ∂t (1) be the heat equation on M, with initial condition f , where κ > 0 is the conductivity and ς is the heat capacity per unit of volume of M. Let t → T(t) be the semigroup associated to the heat equation (1). If M is compact and d ω is the volume element of M, then M T(t) f d ω = M f d ω, (2) for all t ≥ 0. The case in which the manifold M is open was first considered by L. Gårding, when M is an open subset of R n and f is compactly supported. Gårding's proof was generalized by Gaffney in [5] to the case of a manifold M on which the distance d(o, x) of x ∈ M from a fixed point o ∈ M satisfies the inequality M e −ad(o,x) d ω(x) < ∞, ∀ a > 0.(3)In this paper Gaffney's result will be extended to vector-valued differential forms 1 . More specifically, let the Riemannian manifold M be complete and satisfy (3), let E → M be a Riemannian vector bundle on M, and let T be the semigroup defined by the opposite of the Laplace-Beltrami operator whose domain is dense in the Hilbert space of square-summable q-forms on M with values in E (q = 0, 1, . . . , n).If w is a bounded, smooth, E-valued q-form whose differential and codifferential both vanish, thenfor all t ≥ 0 and all smooth, compactly supported q-forms f with values in E.1 Assuming κ = ς = 1.