Legionella pneumophila and Escherichia coli were grown in continuous culture to simulate nutrient-limited, sub-optimal growth conditions that are often experienced in aquatic environments. Washed cultures were treated with chlorine dioxide (C10(2)) or chlorine at a dose equal to 0.75 mg/l. Results showed that the chemostat grown populations of both species were more resistant to inactivation than were their batch-culture grown counterparts. C10(2) was shown to be superior to chlorine on a mass-dose basis in inactivating L. pneumophila. The adequacy of E. coli, a major constituent of the coliform group, as an indicator of Legionella inactivation depended on the antecedent growth conditions: the inactivation behavior of the two organisms grown in continuous culture was similar, whereas L. pneumophila was more resistant than E. coli when both were grown in batch culture.