considering the great ecological and sanitary importance of the cyanobacteria and the need of detailed information about these organisms in brazilian water bodies, the present study aims at contributing towards the knowledge of the cyanobacterial flora of five reservoirs belonging to the upper tietê basin, s？o paulo: billings, guarapiranga, jundiaí, pirapora, ponte nova and taia？upeba. in the past several years, these reservoirs have been submitted to severe environmental deterioration and have repeatedly presented cyanobacterial blooms, including those of toxic species. the samples were collected between 1997 and 2003 either with plankton net (20 μm mesh) or van dorn's bottle, and preserved with lugol solution or formaldehyde. some species were isolated and maintained in culture. forty-eight species of cyanobacteria were identified, with predominance of the order chroococcales (58%), followed by the orders oscillatoriales (21%) and nostocales (21%). among the 48 studied species, 17 (35%) were considered potentially toxic. the occurrence and biodiversity of the cyanobacteria in each reservoir depend on the environmental conditions. among the five water bodies, billings reservoir presented the most adequate situation for the development of a greater number of species (34), probably due to its high ph values (around 8). pirapora reservoir on the other hand, with highest conductivity (445.0 μs cm-1) and lowest secchi depth values (0.2 m), presented the lowest cyanobacterial biodiversity (14 species).