Analysis of a reprocessed seismic reflection line and results from Deep Sea Drilling Project Leg 66 off the southwest coast of Mexico reveal a bottom simulating reflector (BSR) associated with the equilibrium phase boundary of methane hydrate. Several seismic processing techniques were used to accentuate the lateral continuity of the BSR and to delineate the top and base of a 200-700 m thick concentrated hydrate layer. These results suggest the concentrated hydrate layer extends about 20 km parallel to the slope of the inner trench wall in water depths ranging from 2 250-4 500 m. Direct seismic indicators below the BSR and geochemical evidence imply small amounts of free gas may be trapped beneath the base of the hydrate layer and suggest that low-permeability hydrated sediments can act as a seal for reservoirs.