Cold tolerances of juvenile blue tilapiasOreochromis aureusat salinities ranging from 0 to 35%o were determined in the laboratory by decreasing temperatures 1°C/d until fish died. Fish maintained in isosmotic media (11.6%o) survived at lower temperatures than those in water ofhigher or lower salinity. Therefore, the potential range of this exotic species in North America can be expected to extend farthest north in estuarine habitats. Inasmuch as the difference in thermal tolerance between blue tilapias in fresh water and those in isosmotic media was small (about 1°C), the additional habitable range may also be relatively small.