The effective thermal conductivity of snow (keff), which includes latent heat transfer due to vapor diffusion, was measured during three winters in Fairbanks, Alaska. In 1986--1987, keff of several layers of snow was monitored in detail as the snow metamorphosed into depth hoar. Measurements were made using a needle probe with an estimated accuracy of ¿8%; keff was found to decrease and then increase as the snow passed from new snow through several distinct stages of depth hoar. For depth hoar, keff ranged from 0.026 to 0.105, with an average value of 0.063 Wm-1K-1. This is one half to one fourth the value suggested by most studies for snow of similar density. For depth hoar of a given type, keff can be represented as a linear function of temperature between 0¿ and -20 ¿C but requires a nonlinear function for the range from 0¿ to -196 ¿C. At -196 ¿C the thermal conductivity of depth hoar approached that of still air, suggesting that conduction through the ice skeleton of the snow was limited and that the increase in keff at temperatures near 0 ¿C is the result of the strong temperature dependence of water vapor density. This conclusion is consistent with the nature of the ice bonds in depth hoar, which are thin and relatively few in number. ¿ American Geophysical Union 1992