Recent discoveries on the northern Gulf of Mexico continental slope have altered our understanding of biological and chemical processes occurring in the deep ocean. A biological community of hydrothermal vent-type organisms was recently discovered at the base of the Florida Escarpment1, where the fauna are apparently nourished by hydrogen sulphide-rich hypersaline water seeping out onto the sea floor. Dense colonies of deep living chemosynthetic benthic organisms were first discovered during investigations of warm water anomalies along the axis of the Galapagos Rift in the Pacific Ocean in 1977 2 - 4, and this first discovery of clusters of clams, tube worms and other filter feeders in the immediate proximity of warm water vents has been followed by the discovery of a number of other hydrothermal vent sites, for example Guaymas Basin, East Pacific Rise at 21° N. The dense population assemblages at these sites are apparently restricted to small areas of the ocean floor where hydrogen sulphide-rich water is escaping from spreading centres, but the Florida Escarpment discovery indicates that these communities can also exist on passive margins. Here we report the discovery of dense biological communities associated with regions of oil and gas seepage on the Louisiana continental slope. These communities of large epi- and infaunal organisms are similar to those associated with the vents of the Pacific and the hypersaline brine seeps of the Florida Escarpment. Carbon isotope analyses suggest that these communities are chemosynthetic and derive their energy from hydrogen sulphide and/or hydrocarbons. Similar communities may be widely distributed on the sea floor in other oil-producing regions of the ocean. © 1985 Nature Publishing Group.