The goal of this paper was to establish a fundamental understanding of manganese (Mn) in distribution systems with particular emphasis on the utilization of Mn by indigenous bacteria and a strain of manganese oxidizing bacteria (MOB), Leptothrix discophora. Mn causes discoloration of drinking water and will deposit on household fixtures, clothes, and boiled food. Consequently, utilities with chronic Mn episodes will strive for concentrations below the current secondary maximum concentration level (SMCL) of 0.05 mg/L. Studies have identified Mn occurrence by full scale evaluation, but little information is available to utilities for mitigating Mn. Although well documented, MOB's role in Mn accumulation in the distribution system is poorly understood. Two annular reactor (AR) trains containing polycarbonate (PC) coupons and two containing cast iron (CI) coupons are used to model distribution systems. Each train has two ARs in series with retention times of 2 and 6-h, respectively. Influent Mn concentrations were 0.02 or 0.05 mg/L. In one trial no microbial spike was provided to determine background Mn oxidation from Cl₂ and potential biological oxidation from indigenous MOB. The chlorine residual from the first AR was 1.0 mg/L. Results from this study revealed that biological oxidation by indigenous MOB in the heterotrophic bacteria was minimal. Oxidation by residual chlorine was the dominant factor in Mn accumulation in both polycarbonate and cast iron AR trains, for both low (0.02 mg/L) and high (0.05 mg/L) dose of Mn. During the Leptothrix discophora trial there was once again little biological oxidation by heterotrophic bacteria. However, when the Leptothrix discophora were introduced into the system they resulted in significant accumulation in all ARs. After chlorination in this trial the dominant oxidant became the chlorine residual, where a residual was present. As in a full-scale distribution system, both trials predominately had chemical oxidation occur in the first ARs where a residual of chlorine was present. Where there was little to no residual in the second ARs microbial oxidation still dominated. The findings from this research project should help utilities develop better mitigation strategies for manganese occurrence in the distribution system. Includes 27 references, tables, figures.