Anaerobic oxidation of methane (AOM) and sulphate reduction were examined in sediment samples from a marine gas hydrate area (Hydrate Ridge, NE Pacific). The sediment contained high numbers of microbial consortia consisting of organisms that affiliate with methanogenic archaea and with sulphate-reducing bacteria. Sediment samples incubated under strictly anoxic conditions in defined mineral medium (salinity as in seawater) produced sulphide from sulphate if methane was added as the sole organic substrate. No sulphide production occurred in control experiments without methane. Methane-dependent sulphide production was fastest between 4 degree C and 16 degree C, the average rate with 0.1 MPa (approximately 1 atm) methane being 2.5 micro mol sulphide day(-1) and (g dry mass sediment)(-1). An increase of the methane pressure to 1.1 MPa (approximately 11 atm) resulted in a four to fivefold increase of the sulphide production rate. Quantitative measurements using a special anoxic incubation device without gas phase revealed continuous consumption of dissolved methane (from initially 3.2 to 0.7 mM) with simultaneous production of sulphide at a molar ratio of nearly 1:1. To test the response of the indigenous community to possible intermediates of AOM, molecular hydrogen, formate, acetate or methanol were added in the absence of methane; however, sulphide production from sulphate with any of these compounds was much slower than with methane. In the presence of methane, such additions neither stimulated nor inhibited sulphate reduction. Hence, the experiments did not provide evidence for one of these compounds acting as a free extracellular intermediate (intercellular shuttle) during AOM by the presently investigated consortia.