Bacterial populations and activity were quantified at three sites in the Cascadia Margin accretionary wedge, off the West Canadian/American coast (ODP Leg 146). At two sites sediments contained gas hydrates, Site 889/890 had a discrete zone of hydrate approximately 10 m above a bottom simulating reflector (BSR) at 225 m below sea floor (mbsf) and Site 892 had disseminated hydrate in the upper 20 mbsf and a BSR at 74 mbsf. Site 888 was a control site without gas hydrates. The control site (888) and top approximately 90 mbsf of Site 889/890 had bacterial distributions similar to previous Pacific Ocean sites. In the upper approximately 30 m of Site 892, however, bacterial populations were much lower, suggesting inhibition by the high concentrations of H (sub 2) S within the hydrate zone. Below this depth bacterial populations rose to concentrations consistent with other sites. The control site was dominated by SO (sub 4) reduction and rates of CH (sub 4) oxidation in the top 90 m were low (0.002-0.033 nmol cm (super -3) d (super -1) ). At Site 889/890 bacterial populations and activity were stimulated in the discrete hydrate zone. CH (sub 4) oxidation rates increased in the middle of this zone to 134.5 nmol cm (super -3) d (super -1) (ca. 9 times the average rate at other depths), resulting in a significant (X10) increase in the total bacterial population. The anaerobic process(es) responsible for this oxidation remain unclear, despite SO (sub 4) -reducing bacteria, previously associated with CH (sub 4) oxidation, also being stimulated in this zone. Fluid flux into accretionary wedge sediments may be an important process in providing electron acceptors to maintain these relatively high rates of CH (sub 4) oxidation. This first microbiological study of gas hydrates indicates that bacterial processes are influenced by gas and fluid venting, and they play a major role in geochemical changes within these deep (>200 mbsf) sediments.