Fish-killing harmful algal blooms (HABs) threaten fisheries and aquaculture globally. In May and June 2019, the haptophyte Chrysochromulina leadbeateri formed the most devastating fish-killing algal bloom ever recorded in Northern Norway. Despite the severe impact of HABs on farmed fish, our understanding of their toxic mechanisms is limited. Here, we investigated the ichthyotoxicity of C. leadbeateri by using the gill epithelial cell line ASG-10 from Atlantic salmon. Extracts of C. leadbeateri showed potent hemolytic activity towards Atlantic salmon red blood cells, and toxicity to the ASG-10 cells with an EC