Technical Abstract: For most of the thousands of years that humans have used scalding to make it easier to remove feathers from birds, no one knew about the microbiological implications of scalding. The hot water in scald tanks kills many bacteria, but scalding multiple birds in the same tank gives surviving bacteria an opportunity to transfer between carcasses. In multiple-tank, counterflow scalders (where water flows in the direction opposite to movement of carcasses), a decline in numbers and incidence of bacteria in successive tanks has been reported in many studies. Multiple-tank scalders clean up scald water by preventing the most contaminated water in the first tank from mixing freely and spreading throughout the entire volume of scald water in the other tanks, as would happen in a single tank with equivalent total volume of water. Bacteria in a scald tank may come from thousands of different carcasses. A plateau in the numbers of bacteria in scald tanks occurs when the number of dying bacteria is approximately equal to the number of bacteria entering on incoming carcasses. Although it appears that numbers of bacteria suspended in water in multiple-tank scalders can be modeled successfully, it is still not possible to predict how bacteria will move between the feathers and carcass.