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Cylindrospermopsis raciborskii is a freshwater cyanobacterium of tropical origin, also found in temperate re-gions. Due to its known ability to produce potent toxins, such as cylindrospermopsin and paralytic shellfish poisons, this species is of major concern from a water quality perspective. In this article, we report, for the first time, the occurrence of Cylindrospermopsis raciborskii in Tunisian freshwaters. The morphological characteristics of this strain, observed in reservoir Bir M'Cherga, conformed to those of natural population species described in the literature. It produced only straight trichomes. During the study (October 2004-December 2005), the highest density of this species (1.16 × 106 trichomes L-1) was observed in August 2005. The toxicity assessment of one isolated strain was investigated by mouse bioassay, ELISA, HPLC, and molecular biology techniques. In spite of the presence of the PKS gene, the PS and the AMT genes were absent. The Tunisian isolate of Cylindrospermopsis raciborskii from the reservoir Bir M'Cherga does not produce cylindrospermopsin, and it seems not to be a neurotoxin producer. The increased geographical distribution of this planktonic species shows that, due to its invasive behavior, potential risk should be included in the monitoring protocols. [ABSTRACT FROM AUTHOR]