The mean speed of a wrinkled isothermal front is calculated from first principles in terms of the local normal front speed and the intensity of an isotropic, weakly turbulent flow through which the front propagates. It is found that the mean speed of the wrinkled front departs from the local normal front speed (assumed to be the same as the planar-front propagation speed) by an amount proportional to 4/3 of the turbulent kinetic energy of the flow or (equivalently) twice the turbulence intensity.