Oxygen-18 and deuterium abundances vary with depth in ice from a frost blister at a spring site near North Fork Pass, Yukon. They indicate that the ice was formed by continuous downward freezing under nearly ideal, closed-system conditions in a water-filled cavity. Weighted mean values of δ18O and δ2H for the ice samples indicate that the frost–blister ice was formed from groundwater similar to that of nearby springs. The slope of the regression line (5.1) for the 18O and 2H data of the frost–blister ice suggests that nonequilibrium fractionation conditions existed during freezing. Tritium concentrations in the ice show irregular variations with depth; the weighted mean 3H concentration for the ice is, however, similar to that of the spring water.