We studied the planktonic food web in eutrophic San Reservoir (Catalonia, NE Spain). Along the longitudinal axis from the Ter River downstream to the dam, we characterized a microbial succession of food web dominance of bacteria-HNF-ciliates. The Ter River transports a large load of organic material into the reservoir, with a bacterial density of similar to9.10(6) large cells per ml. While at the first lacustrine station of the Reservoir HNF were the dominant bacterial consumers, at the others, an oligotrich ciliate, Halteria grandinella, was the main protozoan bacterivore. Most of the bacterial production in the reservoir epilimnion was consumed by grazing. The spatial succession of the resevoir microbial food webs was followed downstream by maximum densities of their potential predators among zoo-plankters - rotifers, and early developmental stages of copepods.