Cylindrospermopsis raciborskii, a potentially toxic blooming cyanobacterium (blue-green alga), responsible for public health problems in Australia, was identified in France in 1994 in a shallow pond south of Paris. A program monitoring the occurrence of C. raciborskii in this pond was conducted from July 1998 to October 1999. The phytoplankton assemblages were studied, and limnological parameters (water temperature, dissolved oxygen, pH, conductivity, and dissolved inorganic nutrients) were measured. By multivariate analysis (principal component analysis), we showed that sufficiently high temperatures to allow the germination of akinetes, relatively low nutrient concentrations (soluble reactive phosphorus with a mean concentration of 1μM and nitrate between 0 and 5μM, except in February 1999 (21μM)) and a characteristic high and constant sulfate concentration (8981+/-471μM) seemed to be the main factors involved in the proliferation of C. raciborskii in the ''Francs-Pecheurs'' (FP) pond. In the light of these findings and of bibliographic data, C. raciborskii would seem to be characterized by good adaptability, but also by low competitiveness with other phytoplanktonic species in the temperate study area.