Responses of native aquatic bacteria to source and concentration of dissolved organic carbon was observed by using gradient plates. Dissolved organic carbon of prairie (grasses) or gallery forest (bur oak) origin was used as the carbon source in these observations. Bacteria isolated from sediments in the grassland reaches of a prairie stream showed growth only on grass leachates. Bacteria isolated from the gallery forest reaches of the same stream were able to grow on plates made from either source of leachate. The differing quality, quantity, and rate of supply of these dissolved organic carbon sources should select for variation in the baterial assemblages