Abstract. We present a rigidity property of holomorphic generators on the open unit ball B of a Hilbert space H. Namely, if f ∈ Hol(B, H) is the generator of a one-parameter continuous semigroup {F t } t≥0 on B such that for some boundary point τ ∈ ∂B, the admissible limitLet H be a complex Hilbert space with inner product ·, · and induced norm · . If H is finite dimensional, we will identify H with C n . We denote by Hol(D, E) the set of all holomorphic mappings on a domain D ⊂ H which map D into a subset
E of H, and put Hol(D) := Hol(D, D).We are concerned with the problem of finding conditions for a mapping F ∈ Hol(D, E) to coincide identically with a given holomorphic mapping on D when they behave similarly in a neighborhood of a boundary point τ ∈ ∂D.A number of basic results in this direction are due to D. M. Burns and S. G. Krantz [6]. They establish conditions at a boundary point for a holomorphic self-mapping F of the open unit disk ∆ := {z ∈ C : |z| < 1} to coincide with the identity mapping (see Proposition 1 below). Then they generalize this fact to the n-dimensional case: for holomorphic self-mappings of the open unit ball (see Proposition 3 below) and of strongly pseudoconvex domains in C n . Further developments of this theme are presented by X. J. Huang in [15], where he obtains similar results for weakly pseudoconvex domains. More recently, L. Baracco, D. Zaitsev and G. Zampieri [3] have proved local boundary rigidity theorems for mappings defined only on one side as germs at a boundary point, and extended their results from boundaries of domains to submanifolds of higher codimension. More higher-dimensional results can be found, for instance, in [2] and [11].In this paper we present a rigidity theorem for holomorphic generators on the open unit ball B of a Hilbert space H which generalizes the analogous theorem for the one-dimensional case [8,17,7] and properly contains the above-mentioned Burns-Krantz theorem for the open unit ball in C n .