The St. Louis Metropolitan Area surrounds the confluence of two of America's largest rivers, the Mississippi and the Missouri. The Mississippi is, in fact, so large at St. Louis, and downstream, that it has gained the reputation of being practically immune to damage by pollution. This reputation is not justified, as is shown by the report of the Bi-State Development Agency, Mississippi River Water Pollution Investigation, St. Louis Metropolitan Area, published in 1954 in cooperation with the States of Illinois and Missouri and the U. S. Public Health Service. The present report draws heavily on that excellent publication for basic data, facts, and conclusions. For the report data on sources of waste have been brought up to date and additional data including especially those on bacterial quality from several water supplies, have been assembled. Additional computations, based on the 1951-1952 data on bacterial contamination, oxygen demand, and dissolved oxygen both support and supplement the conclusions of the 1954 report.