The. translational self-diffusion coefficients of supercooled water at atmospheric pressure were examined using pulsed-gradient spin-echo NMR diffusion measurements down to 238 K. As the temperature decreased, the diffusion behavior became distinctly non-Arrhenius. It was found that the diffusion behavior when plotted in an Arrhenius form was well-described by a Vogel-Tamman-Fulcher-type relationship in the temperature range from 298 to about 242 K. However, a fractional power-law-type equation was found to provide a better fit that extended over the entire measured temperature range. Below this temperature range, the diffusion coefficient decreased rather steeply, and at 238 K, the diffusion coefficient was 1.58 x 10(-10) m(2) s(-1), the lowest value of the water diffusion coefficient so far determined. At this temperature the activation energy for the diffusion was found to be of the order of 44.4 kT mol(-1). The data presented here should allow theoretical models of water to be more stringently tested.