Species of the harmful algal bloom (HAB)-forming genus Chattonella (Raphidophyceae) are responsible for massive fish kills worldwide. Despite this, the genus remains still unexplored in Argentinean waters in the Southwestern Atlantic. Three Chattonella strains (LPCc045, LPCc046, and LPCc047) were isolated and cultured as part of a harmful phytoplankton monitoring program in the marine coastal waters of Buenos Aires Province to determine: 1) taxonomic and genetic identification, 2) pigment compositions, and 3) ichthyotoxic potency and ROS production. Morphological observations (light and transmission electron microscopy) matched classical descriptions of C. marina. Phylogenetic reconstruction using a partial sequence of the ribosomal gene (LSU D1D2) confirmed the cluster of the three strains within the C. marina complex clade (99 % bootstrap support). Pigment analyses by HPLC showed that the most abundant was chlorophyll-a, followed by fucoxanthin and other carotenoids. Gill cell damage, used as a proxy of cytotoxicity, was assessed using the rainbow trout RTgill-W1 cell-based assay against cell supernatant and intracellular treatments. Cytotoxic potency was positively correlated with cell abundance, with supernatant treatment causing more gill cell damage than intracellular content, reducing gill cell viability down to 60 % of control at 1.9 x 103 cells mL-1. Superoxide production was comparable with other globally distributed strains (supernatant 7.63 +/- 0.67 pmol O2- cell-1 h-1; intracellular 8.1 +/- 0.56 pmol O2- cell-1 h-1). This study provides the first morphological, phylogenetic, physiological and toxicological characterization of the fish-killing algae C. marina complex in Argentinean waters.