  The study is devoted to radial pulsations in Population I Cepheids with
masses from 5.4 to 6 Msun. Solution of the equations of radiation hydrodynamics
and turbulent convection for nonlinear stellar pulsations was obtained with
initial conditions as the core helium burning models of computed evolutionary
sequences. For each value of the initial mass we considered stellar evolution
from the zero age main sequence to central helium exhaustion with three values
of the convective overshoot parameter: aov=0.1, 0.15 and 0.2. Models for the
Cepheid SZ Tau with pulsation period 3.149 day can be constructed only for
aov=0.1 and aov=0.15. The star is at the evolutionary stage of the second
crossing of the instability strip and pulsates in the first overtone just near
the boundary between the fundamental mode and the first overtone. The mass,
radius and age of the star are in the ranges 5.46 <= M/Msun <= 5.75, 41.5 <=
R/Rsun <= 42.3 and 6.9e7 yr <= t <= 8.0e7 yr, respectively. Predicted period
change rates are of -0.4 s/yr.
