The principal weakness in the performance evaluation of water treatment filtration technology for particle removal to date has been the failure to characterize the particles in suspension in influent and treated water. The direct microscopic count may be used in water treatment plants to evaluate total bacterial removals, observe seasonal (temperature) effects on water treatment plant process efficiency and estimate the removal of planktonic versus particle-associated bacteria. Comparisons between the direct microscopic count and turbidity, total coliform, heterotrophic plate count and electronic particle counting are made based on recent studies at the Capital City Water Treatment Plant, Jefferson City, Missouri. The direct microscopic count yields information on the reduction of total bacterial cells, planktonic and particle-associated bacteria, algal cells, nematodes and total number of particles larger than 3.0 μm. The removal of the turbidity causing solids does not correspond to the reduction of biotic particles. As a result there are marked differences in turbidity and microbial reductions during treatment. The direct microscopic count has been judged to provide one of the most basic and fundamental measures of bacterial ecology because it provides an estimate of total bacterial population. Since it has been previously demonstrated to be rapid, specific, sensitive and accurate, it has now begun to find application in observing the removals of a wide range of particles during water treatment.