The biological community in activated sludge wastewater plants is organized within this ecosystem as bioaggregates or flocs, in which the biotic component is embedded in a complex matrix comprised of extracellular polymeric substances mainly of microbial origin. The aim of this work is to study the role of different floc-associated ciliates commonly reported in wastewater treatment plants-crawling Euplotes and sessile Vorticella- in the formation of aggregates. Flocs, in experiments with ciliates and latex beads, showed more compactation and cohesion among particles than those in the absence of ciliates. Ciliates have been shown to contribute to floc formation through different mechanisms such as the active secretion of polymeric substances (extrusomes), their biological activities (movement and feeding strategies), or the cysts formation capacity of some species. Staining with lectins coupled to fluorescein showed that carbohydrate of the matrix contained glucose, manose, N-acetyl-glucosamine and galactose. Protein fraction revealed over the latex beads surfaces could probably be of bacterial origin, but nucleic acids represented an important fraction of the extracellular polymeric substances of ciliate origin.