The results of a field experiment that investigated the in situ association of total coliforms (TC) and E. coli (EC) with suspended inorganic particles in a drinking water reservoir are presented. The experimental program measured TC and EC at various locations and at multiple depths in a medium sized Australian reservoir subject to continuous inflow forcing. Particle concentrations and size distributions were measured using a LISST (Laser In Situ Scattering Transmissometer) profiler. Correlations between the particle measurements and the bacteria were calculated to provide in situ evidence for the association of TC and EC with suspended inorganic particles. Both TC and EC correlated most strongly with fine particles between 3.2 and 4.5 mu m. Development of a simple Lagrangian model of the inflow for particles and microbes additionally provided insight into the relative roles of dilution, sedimentation, and inactivation for the bacteria. The model results and the correlations support the theory that high association with the small particles was due to their high number concentration and the effective surface area available for attachment. It was also evident that the majority of bacteria (> 80%) were physically associated with the suspended particles and allowed a quantitative estimate of the net sedimentation rate. Bacterial attachment and the concomitant increase in settling should be considered when modeling bacterial dynamics, during design of monitoring programs and when implementing pathogen risk management strategies.