Evidence is presented of the occurrence of sulphate-reducing bacteria in salt waters associated with petroleum in six producing wells in the Lawrenceville, Illinois, region. Five were producing from the Bridgeport sand from depths between 900 and 1,000 feet and one from the McClosky sand at 1,850 feet. All of these brines carried hydrogen sulphide. Brines from three other oil wells were free from hydrogen sulphide and were free also from sulphate-reducing bacteria, suggesting that bacteria may play the major role in the formation of the hydrogen sulphide associated with the petroleum.

As bearing on the possibility of bacterial contamination of oil wells from surface sources subsequent to drilling, eight soil samples, eight fresh-water well samples and one river-water sample were collected in the Lawrenceville and Allendale regions. Seven of the eight soils and all of the fresh well waters were free from sulphate-reducing bacteria. These results lend no support to the view that the abundant bacteria of the oil-well brines came from such sources after the wells were drilled.