The bacteriological analysis of 126 sediment samples collected in the Channel Island region along the coast of Southern California from bottoms as deep as 2,000 meters reveals the presence of several physiological types of bacteria which may influence the diagenesis of recent sediments. Strict aerobes recovered from the bottoms of cores over 50 cm long where there is no free oxygen may have been buried in the sediments in a dormant state for a long time. Core depth, organic-matter content, and the median particle size of the sediments are the chief factors which influence the bacterial populations. Most of the bacteria are capable of multiplication and biochemical activity at 0 degrees C. Bacteria which decompose proteins, cellulose, starch, chitin, and other complex organic compounds are quite abundant in the bottom deposits. The occurrence of lipoclastic species which utilize the glycerol from fats and leave long-chain fatty acids may help account for the genesis of petroleum.