  We have studied the resistivity, $\rho$, of a two-dimensional electron system
in silicon in the temperature range 200 mK < T < 7.5 K at zero magnetic field
at low electron densities, when the electron system is in the insulating
regime. Our results show that at an intermediate temperature range $\rho=\rho_0
exp[(T_0/T)^{1/2}]$ for at least four orders of magnitude up to 3x10^9 Ohms.
This behavior is consistent with the existence of a Coulomb gap. Near the
metal/insulator transition, the prefactor was found to be close to $h/e^2$, and
resistivity scales with temperature. For very low electron densities, $n_s$,
the prefactor diminishes with diminishing $n_s$. A comparison with the theory
shows that a specific set of conditions are necessary to observe the behavior
of resistivity consistent with the existence of the Coulomb gap.
