The study of phytoplanktonic organisms has grown in recent times, as a consequence of its importance in several aquatic ecosystems, both in continental and marine environments. In stabilization ponds, for example, they act in the biological treatment of sewage. These ponds are an eutrophic environment, characterized by a considerable growth of phytoplankton. To study the phytoplankton composition in such ponds, samples were collected in a facultative pond in a sewage treatment station, located in Barbalha (Ceará State, Brasil), from March/2006 to January/2007. The samples were analyzed in vivo and fixed, then indexes of abundancy, frequency, diversity and equitability were calculated. Cyanophyta, Euglenophyta, Bacillariophyta and Chlorophyta species were identified. The most abundant species and present in all samples were Planktothrix isothrix (Skuja) Komárek et Komárková. and Microcystis aeruginosa (Kütz) Kütz., which are both toxic species. In both dry and rainy seasons, variation of the phytoplankton composition wasn’t observed and the high presence of P. isothrix resulted in a low diversity. The phytoplankton groups observed are typical of eutrophic environments.