Cylindrospermopsis raciborskii, a freshwater cyanobacterium of tropical origin, is not only increasingly found in (sub)
tropical water bodies, but also in temperate regions. Since this species may produce potent toxins such as cylindrospermopsin
(CYN) and paralytic shellﬁsh poisons, its massive occurrence in water bodies used as drinking water sources or for recreation is
of major concern. The proliferation of C. raciborskii in German water bodies has been documented for the past decade. We
investigated the occurrence of CYN in ﬁeld populations and isolates of C. raciborskii from two lakes, and assessed the toxicity
of culture isolates using the mouse bioassay, primary rat hepatocytes and human derived cell lines.
We show for the ﬁrst time the occurrence of CYN in German water bodies. None of seven isolates of C. raciborskii contained
CYN, however, all isolates were toxic to primary rat hepatocytes, human hepatoblastoma (HEP-G2) and human colon
adenocarcinoma (CACO-2) cells. Methanolic extracts were more toxic than aqueous extracts. Three isolates tested in the mouse
bioassay were toxic at a concentration of 800 mg kg^-1 showing liver and spleen damage and inﬂammation of the intestine.
These results give strong evidence that the German isolates of C. raciborskii contain currently not identiﬁed or unknown toxins.