| Summary: | In the summer of 2015, an intense cyanobacterial bloom producing geosmin/2-methylisoborneol (MIB) occurred in the Roxo freshwater reservoir in Alentejo, Portugal. The drinking water supplied from the Roxo water treatment plant (WTP) exhibited an unpleasant odor/taste and a significant cyanobacteria density was detected in the finished water at the exit of the WTP. Cyanobacteria were not evaluated downstream of the WTP, namely, at the city reservoir. The aim of this work was to isolate and characterize viable cyanobacteria present in finished water (exit of the WTP and city reservoir) that withstand conventional water treatment. Treated water samples collected at both sites were inoculated in Z8 culture medium to provide the conditions for putative cyanobacterial growth. After 30 days, filamentous cyanobacteria were observed in cultures inoculated with samples from the exit point of the WTP. Viable trichomes were isolated and identified as <i>Cylindrospermopsis raciborskii</i> by morphometric and molecular analysis. None of the isolates were cylindrospermopsin/microcystin producers, as confirmed by ELISA and amplification of corresponding genes (<i>PS</i>/<i>PKS</i> and <i>mcyA-cd</i>/<i>mcyAB</i>/<i>mcyB</i>). ELISA results were positive for saxitoxin, but saxitoxin and derivatives were not detected by liquid chromatography with fluorescence detection (LC-FLD), nor were their related genes (<i>sxtA</i>/<i>sxtA4</i>/<i>sxtB</i>/<i>sxtM</i>/<i>sxtPer</i>/<i>sxtI</i>). To our knowledge, this is the first report on the establishment of cultures of <i>C. raciborskii</i> that resisted water treatment processes.
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