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 1 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 1 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 DH2 and 1 8H2 values of 1 and 2.2%o, respectively.