Molecular and isotopic measurements of gas and water obtained from a gas hydrate at Site 570, DSDP Leg 84, are reported. The hydrate appeared to be Structure I and was composed of a solid framework of water molecules enclosing methane and small amounts of ethane and carbon dioxide. Carbon isotopic values for the hydrate-bound methane, ethane, and carbon dioxide were - 41 to about - 44, - 27, and - 2.9%o, respectively. The δ1 3CCi values are consistent with void gas values that were determined to have a biogenic source. A significant thermogenic source was discounted be cause of high C1/C2 ratios and because the δ1 3CCC 2 values in these sections were also anomalously heavy (or more positive) isotopically, suggesting that the methane was formed biogenically by reduction of heavy CO2. The isotopically heavy hydrate <513CC2 is also similar to void gas isotopic compositions and is either a result of low-temperature diagenesis producing heavy C2 in these immature sediment sections or upward migration of deeper thermogenic gas. The salinity of the hydrate water was 2.6‰ with δDH2 and δ1 8θH2 values of + 1 and + 2.2%o, respectively.