The forest floor of black spruce (Picea mariana (Mill.) B.S.P.) stands is typified by a thick layer of moss and organic material overlying the soil. This layer is an important source of nutrients and water for spruce and provides thermal insulation of permafrost. However, little is known about the water retention characteristics and thermal conductivity of black spruce forest floors. Near Fairbanks, Alaska, in situ thermal conductivity and water content of a black spruce forest floor (0-0.05 m depth) were assessed during the 1992 growing season. In addition, core samples were taken for laboratory estimates of bulk density, water retention, and thermal conductivity. Bulk density was about 0.035 Mg m-3 at the 0 to 0.10-m depth and 0.050 Mg m-3 at the 0.10 to 0.20-m depth. In situ thermal conductivity of moss was generally smaller than previously estimated and ranged from 0.03 to 0.09 W m-1 K-1 over the growing season. Thermal conductivity varied linearly with water content at the 0 to 0.10-m depth and exponentially with water content below 0.10 m. Water retention differed within the forest floor profile, with an apparently smaller air-entry matric suction and pore size distribution index below the 0.1-m depth. This study suggests that variations in bulk density within a forest floor profile, which result from changes in organic material composition, affect both the thermal and hydrologic properties of black spruce forest floors.