Survival of five species of bacteria was determined in freshwater microcosms containing stream water, leaves, and sediments. Native stream species (Burkholderia cepaciaandAeromonas caviae) became established in sediment biofilms, but coliforms (Citrobacter freundiiandEscherichia coli) and an insecticide-producing bacterium,Bacillus thuringiensis, did not.A. caviaeandB. thuringiensiswere common on leaf surfaces, but population sizes were much lower than those ofB. cepacia. Population sizes of all species decreased substantially in the water column over the course of the seven day experiments. To examine the basis of this high mortality in the water column, the effects of predation, competition, and resource limitation were examined. Competition and predation attributable to smaller microorganisms (< 1μm) had the greatest impact on bacterial mortality. These results suggest that the overall significance of benthic habitats as sources of stream bacteria was enhanced by high rates of mortality in the water column. Bacteria that enter streams through human-mediated means (sewage in the case of coliforms, and insecticide application in the case ofB. thuringiensis) differed in their survival; biofilms enhanced the success ofB. thuringiensisbut not the coliforms.