Cylindrospermopsis raciborskii, a blooming species of cyanobacteria (blue-green algae) was first identified in France in 1994 from a fishing pond localized in the south Paris area (Viry-Châtillon). The geographic distribution of this species first recorded in tropical to subtropical areas (Indonesia, 1913, India, 1912, Australia,1979), is now spread to temperate countries, and this french localization is among the most septentrional ones. In november 1979, an outbreak of severe hepatoenterities and renal damage involved 148 people among an Aboriginal population in Palm Island (North Queensland, Australia). A new toxin, cylindrospermopsin, was purified from a C. raciborskii strain, hold responsible for the disease. Then other toxicity episodes due to C. raciborskii were observed in Australian freshwater reservoirs and recreational waters. More recently, paralytic shellfish poisons (PSPs) were identified in two Brazilian C. raciborskii strains. Public health problem raised by C. raciborskii proliferations led us to develop a monitoring program on the occurrence of C. raciborskii in Viry-Châtillon pond. Since july 1998 and every month, the variations of phytoplanktonic populations were studied and the physical and chemical parameters measured. After a positive mouse bioassay, bloom extracts were screened systematically for cylindrospermopsin and for PSPs, toxins potentially produced by C. raciborskii strains: cylindrospermopsin was analyzed by High Performance Liquid Chromatography coupled to a ultra-violet diode-array detector (HPLC-DAD), and PSPs by both the mouse neuroblastoma cell bioassay and HPLC with postcolumm oxidation followed by fluorescence detection.This monitoring program allowed the determination of C. raciborskii proliferation conditions, and the characterization of its in situ toxic potential. Besides, laboratory cultures have been carried out to determine taxonomical, physiological and toxicological properties of the strains isolated from blooms.