Abstract Juvenile cobias Rachycentron canadum were exposed to decreasing salinity (∼1‰ per day) in a series of five experiments to assess their low-salinity resistance. All fish survived for 24 h at a salinity of 2‰, but 73% died within 24 h of a reduction to 1‰ salinity. Plasma osmolality decreased significantly with decreasing salinity. Fish held for 7 d at 2, 5, or 8‰ after an approximately 1‰ daily reduction differed significantly in plasma osmolality; fish held at a salinity of 2‰ exhibited significantly lower plasma osmolality than fish in the other treatments. Doubling the environmental Ca concentration did not affect plasma osmolality. The results indicate that cobias have limited euryhaline abilities but not enough to sustain low-salinity culture operations.