ABSTRACT Sediment containing gas hydrates from two distant Deep Sea Drilling Project sites (565 and 568), located about 670 km apart or the landward flank of the Middle America Trench, was studied to determine the geochemical conditions that characterize the occurrence of gas hydrates. Site 565 was located in the Pacific Ocean offshore the Nicoya Peninsula of Costa Rica in 3,111 m of water. The depth of the hole at this site was 328 m, and gas hydrates were recovered from 285 and 319 m. Site 568 was located about 670 km to the northwest offshore Guatemala in 2,031 m of water. At this site the hole penetrated to 418 m, and gas hydrates were encountered at 404 m. Both sites are characterized by rates of sedimentation exceeding about 30 m/m.y. and organic carbon contents exceeding about 0.5%. The magnitudes and trends of gas compositions, residual gas concentrations and chlorinity variations are generally similar at both sites. The major organic geochemical differences at these sites appears to be in the carbon isotopic composition of methane [δ13 C(CH4)] and pore water ΣCO2[δ13C(ΣCO2)] at depth. The carbon isotopic compositions are significantly heavier at Site 568 than at Site 565. For example, δ13C(CH4) values at sites 565 and 568 are about −64 and −40 per mil, respectively; δ13C(ΣCO2) values are about +17 and +35 per mil. The isotopic compositions and trends at Site 565 are typical of biogenic methane generation. At Site 568, the isotopic compositions are very heavy, reflecting an unusual source of ΣCO2. In spite of its heavy carbon isotopic composition, this methane is believed to have a biogenic source.