  We compute the phase lags between the radial velocity curves and the light
curves $\Delta \Phi_1= \phi^{V_r}_1 - \phi^{mag}_1$ for classical Cepheid model
sequences both in the linear and the nonlinear regimes. The nonlinear phase
lags generally fall below the linear ones except for high period models where
they lie above, and of course for low pulsation amplitudes where the two merge.
The calculated phase lags show good agreement with the available observational
data of normal amplitude Galactic Cepheids. The metallicity has but a moderate
effect on the phase lag, while the mass-luminosity relation and the parameters
of the turbulent convective model (time-dependent mixing length) mainly
influence the modal selection and the period, which is then reflected in the
period -- $\Delta \Phi_1$ diagram. We discuss the potential application of this
observable as a discriminant for pulsation modes and as a test for ultra-low
amplitudes (ULA) pulsation.
