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From November 1992 to October 1993, Billings Reservoir in São Paulo State was analyzed to identify its environmental quality. Several physical, chemical, biological, and microbiological parameters were evaluated, phytoplankton being among them. Phytoplankton samples were taken monthly at eight sites, including one located on the Rio Pequeno arm. From the samples taken at Rio Pequeno, a total of 69 taxa were recorded. Cylindrospermopsis raciborskii (Wolosz.) Seenaya and Subba Raju was the dominant species from November 1992 to May 1993. Then Scenedesmus quadricauda (Turp.) Bréb. and S. opoliensis P. Richt (Chlorophyceae) became dominant. Another Cyanophyceae that was frequent but less expressive in terms of density and biomass was Oscillatoria sp. Physical and chemical water parameters, including pH, dissolved oxygen, transparency (Secchi disk), conductivity, water and air temperatures, depth, total organic nitrogen, nitrate, total phosphorus, aluminum, and iron also were analyzed. Water temperature, pH, dissolved oxygen, and depth were positively correlated with C. raciborskii density, while aluminum was negatively correlated. S. quadricauda and S. opoliensis densities were inversely correlated to the same factors. There was no indication of correlation between Oscillatoria sp. density and the abiotic factors analyzed. C. raciborskii seems to be favored by physical and chemical characteristics of Rio Pequeno, such as high pH and high levels of iron, and also by biological characteristics such as the capability to migrate on the water column and to fix atmospheric nitrogen. The species toxicity is another possible explanation for the dominance of C. raciborskii, through the growth suppression of other phytoplanktonic groups and protection against grazing by zooplankton. The toxicity of these algal blooms could be responsible for fish kills which occurred at the same place in 1990. C. raciborskii dominance at Rio Pequeno during most of the year advises against the use of this water for domestic supply, due to its potential toxicity, which represents risks to human health, requiring appropriate treatment. © 1998 John Wiley & Sons, Inc. Environ Toxicol Water Qual 13: 73–81, 1998