Let k > 0 be an integer, T = (T(t)), e n a Co-group of bounded operators and A the infinitesimal generator of T. We prove that if,and if the spectrum of A is equal to [X], then A is bounded and (A -k) k+l = 0. Examples are given to show that these conditions are, essentially, the best possible.1991 Mathematics subject classification (Amer. Math. Soc): 47D03.