The cyanobacterium Aphanizomenon favaloroi was identified for the first time in Europe in the Mediterranean brackish Lake Vistonis during July–August 2014. It formed a dense bloom (from 16×106 to 81×106 trichomes L–1), causing a brown colouration of the lake water. When A. favaloroi formed 100% of the total phytoplankton biomass (44mg L–1), saxitoxins (saxitoxin and neo-saxitoxin) were detected in the lake seston (42 and 17μg g–1 phytoplankton dry weight respectively), which was screened for saxitoxins, cylindrospermopsin, anatoxin-a, microcystins and nodularin. A massive fish kill coincided with the A. favaloroi bloom. This new saxitoxin-producing species of cyanobacteria, with traits for successful dispersal, may pose a health risk to animals and humans and cause adverse effects on water quality and water services because of its expansion potential.