The processes of methane oxidation and sulfate reduction were examined in subsamples of gas hydrate associated materials collected along the Gulf of Mexico continental slope. Standard radiotracer techniques were used to determine rates of microbial activity in different layers of the hydrate environment, including outer sediment (OS), interface sediment (IS), worm burrow sediment (WB), interior hydrate (IN) and a mixture of hydrate and sediment (MIX). The anaerobic oxidation of methane (AOM) and sulfate reduction (SR) were observed in all hydrate samples examined and the rates of these processes showed similar spatial trends between different hydrate layers. Highest rates of both AOM and SR were observed at interface between the sediment and hydrate. AOM rates were about 3-11 nmol cm-3 day-1 in worm burrow and interface sediments as compared to < 1 nmol cm-3 day-1 in other hydrate material types. Rates of SR ranged from 59 to 490 nmol cm-3 day-1 in worm burrow and interface sediments while rates in interior hydrate samples were an order of magnitude lower. These rates observed in hydrate materials are lower than rates from nearby methane-rich sediments at ambient temperatures. Nevertheless, our data show that active microbial populations inhabit all layers of the hydrate environment and suggest their activity may impact biogeochemical methane and sulfur cycling in this unique niche. © 2004 Elsevier B.V. All rights reserved.