Abstract.We consider a second-order differential equation periodic in t with period 7" > 0 and with linear damping. Bounds are given for the derivative of the restoring force wnich will guarantee the existence and uniqueness of a /"-periodic solution such that the unique 7"-periodic solution is asymptotically stable. These conditions also rule out the existence of additional periodic solutions which are subharmonics of order 2 .In this note we consider periodic solutions of the differential equation (1) u +ku + g(t, u) = 0 where zc > 0 is a constant, g and its partial derivative with respect to the second variable, denoted by D2g, are continuous, and g is F-periodic in t for some T > 0 . Our goal is to find conditions of the form (2) a < D2g(t, Í) < b for (t, ¿;) e R which will guarantee the existence and uniqueness of a Tperiodic solution u0 which is locally, exponentially, asymptotically stable, i.e. such that there exist constants C > 0 and a > 0 such that if u is another solution with |m(0) -m0(0)| and \u (0) -u'0(0)\ sufficiently small, then \u(t) -u0(t)\ < Cde~at, \u'(t) -u'0(t)\ < Cde~nt for all t > 0, where d = |«(0) -The existence and uniqueness part of this problem has been considered in several papers when zc = 0. In this case, if there exists an integer TV > 0 such that 4n2N2/T2 < a < b < 4n2(N + \)2/T2 and (2) holds, then there exists a unique F-periodic solution of ( 1 ). This follows from work of Loud [6], under the additional assumption that a certain symmetry condition holds, and