In spite of reliance on Escherichia coli as an indicator of fecal pollution in water resources, including potable water, relatively little is known of its ecology and persistence. It is particularly important to be able to accurately detect E. coli presence and quantity in treated potable water because of potential human health effects from consumption of contaminated water. Presence of even a single E. coli in potable water can lead to significant consequences and costs for water utilities. Alum is frequently used as a coagulant in conventionally treated water, and is usually present in slight excess after treatment. We show here that E. coli can accumulate the Al3 +  ions present at natural levels in potable water and in so doing become uncultivable. Thus, E. coli, and perhaps a number of other bacteria, present in potable water could readily escape detection on laboratory culture media. Chelation with Tiron of the Al3 +  prior to exposure maintained the E. coli in a cultivable state. This phenomenon deserves further investigation in view of the reliance placed on E. coli as an indicator of fecal contamination.