The evolution of integrated pulse profiles from high to low frequencies does not conform exactly to the simple model of a dipole field. Several groups (Davîes et al. 1984, Kuz'min 1986, Shitov, Malofeev, and Izvekova 1988) have suggested large field twisting due to the rotation of the pulsar, which leads to a delay in pulse arrival times for low frequencies whose sources are located in the outer parts of the magnetosphere. We are, therefore, comparing profiles of about 30 pulsars, observed at frequencies from 40 to 1400 MHz. Our preliminary analysis shows that significant delay at low frequencies occurs in many pulsars and we present here some typical examples.