Chloride concentrations in surface waters have increased significantly, a rise attributed to road salt use. In Canada, this may be a concern for endangered freshwater mussels, many with ranges limited to southern Ontario, Canada’s most road-dense region. The acute toxicity of NaCl was determined for glochidia, the mussel’s larval stage. The 24h EC50s of four (including two Canadian endangered) species ranged from 113–1430mgClL −1 (reconstituted water, 100mg CaCO 3 L −1 ). To determine how mussels would respond to a chloride pulse, natural river water (hardness 278–322mg CaCO 3 L −1 ) was augmented with salt. Lampsilis fasciola glochidia were significantly less sensitive to salt in natural water (EC50s 1265–1559mg Cl L −1 ) than in reconstituted water (EC50 285mgL −1 ). Chloride data from mussel habitats revealed chloride reaches levels acutely toxic to glochidia (1300mgL −1 ). The increased salinization of freshwater could negatively impact freshwater mussels, including numerous species at risk.