A thermal penetrator that was air dropped into a freshly emplaced pumice flow at Mt. St. Helens yielded information on the in-situ thermal properties of the pumice. The in-situ conductivity-density-specific heat product at a depth of 60 cm was found to be 7.24¿10-5 cal2cm/4 s- ¿C2 at an average pumice temperature of 200 ¿C. Using this data, values for the average in-situ thermal conductivity (2.9¿10-4 cal/cm-s-¿C) and thermal diffusivity (1.2¿10-3 cm2/s) were estimated. These thermal properties are of use in studies of pumice cooling and in the interpretation of infrared remote sensing data.