A laboratory investigation was used to determine whether biogenic gas bubbles accumulate and block water-conducting pores below the water table in poorly decomposed Sphagnum peat. We found that biogenic gas bubbles did accumulate under realistic incubation temperatures. At the end of incubations at 10.5 ¿C, volumetric water contents in two peat samples decreased to between 0.8 and 0.85 (porosity of the samples ranged from 0.96 to 0.97), indicating that the peat was considerably undersaturated with respect to water. Methane was found to be an important constituent of the gas bubbles. The presence of gas bubbles appeared to have a major effect on hydraulic conductivity (K). In control incubations, prior to which the peat had been irradiated and dosed with a biocide, biogenic gas bubbles did not accumulate, and K was 5--8 times higher than at the end of the microbially active incubations. Our results suggest that biogenic gas bubbles have a potentially significant effect on seepage in peat soils. ¿ 2001 American Geophysical Union