This paper reports some theoretical and experimental investigations on the propagation of a pulse in long step-index optical fibers where mode conversion is present. We derive simple analytical expressions for the frequency response and the radiation pattern as a function of the fiber's length provided the launching conditions are known. A comparison with experimental observations made on a 3-km long Corning fiber shows reasonable agreement. We conclude that the approximations used to obtain the simple analytical solutions are satisfactory for predicting the transmission characteristics of the fiber.