We shall consider a one-dimensional semilinear wave equation in a finite interval (0, l) in R. A usual homogeneous boundary condition is imposed on it at x = 0, l. We shall show that the solution u(x, t) oscillates on the time t in the following sense. Let x be any fixed in (0, l), then u(x, t) changes its sign infinitely many times as t is passing. (C) 2003 Elsevier Ltd. All rights reserved.