Petroleum of apparent hydrothermal origin has been reported among samples of hydrothermal mounds encrusted with tube worms, massive sulfides, and other hydrothermal mineral assemblages dredged from the Guaymas Basin (Nature, 295, 198–202). The petroleum contains a set of diagnostic organic indicators that could not have been dissolved in mature petroleum migrated to the hydrothermal site from geologically older formations. On the other hand, the petroleum samples contain sufficient chemical evidence to prove an autogenic origin. Researchers B. Simon of the Institute of Geophysics and Planetary Physics and P. Lonsdale of the Marine Physical Laboratory, Scripps Institution, suggest that the oil in the Guaymas Basin formed in situ from immature organic matter by thermal alteration from the vents. The hydrothermal mineralized mounds along the southern rift of the Guaymas Basin are being explored for their potential sulfide ores. This discovery of petroleum produced by the same process constitutes a major scientific step in the study of petroleum formation. The organic matter could be an essential carbon source for microbes involved in a symbiotic process that supports higher life forms, such as tube worms, and simultaneously precipitates sulfides. The study consisted of careful dredge sampling by the crew of the research vessel Melville, selection of specimens from the dredge, and chemical analysis by gas chromatographic and mass spectrometric techniques. The analyses were thorough and contain extensive parameters to prove a thermogenic origin of the petroleum material.