A survey (7 months) has been made of the distribution of B. coli and B. aerogenes in different units of a sewage disposal plant. Numbers in the incoming sewage were lower in winter. Whereas a maximum reduction of solids took place in the Imhoff tanks, a consistent bacterial reduction here did not always occur. In the filter beds, with smaller reduction of solids, there was a maximum reduction of bacteria. Numbers of B. coli and B. aerogenes in the digestion of unseeded fresh solids under laboratory conditions increased to a maximum within 2 days, after which they fell rapidly to a low level. A similar relationship holds when fresh solids are added daily to ripe sludge, with the difference that the joint of maximum numbers is delayed. Incidence of rapid rise and fall of these organisms with initial increase in acidity has been attributed to a causal relationship.