Under conditions commonly found in Earth's atmosphere, water can form two solid phases; hexagonal ice (Ih) and cubic ice (Ic). Recent reports have suggested that Ic may form in the atmosphere under a wider range of conditions than previously believed. In light of these reports, the formation of Ic has been suggested as one contributing factor for in-situ observations of persistent in-cloud supersaturations in cold cirrus. However, an accurate evaluation of the contribution of Ic formation to the observed supersaturations requires knowledge of the saturation vapor pressure of Ic, which has not been measured. In this manuscript, we report direct measurements of the vapor pressure of Ic over the temperature range 180--190 K. Over this temperature range, the vapor pressure of the cubic phase is 10.5 ¿ 2.5% higher than that of the hexagonal phase. Field measurements of in-cloud supersaturations made during CRYSTAL-FACE are also re-analyzed and discussed.