1. Salinity tolerance, osmoregulatory mechanisms and some effects of salinity on embryonic development and metamorphosis have been studied in the tadpoles of the euryhaline crab-eating frog (Rana cancrivora).
2. Tadpoles in developmental stages III-XIX survive well in all salinities from fresh water to 32% sea water. They are good osmoregulators over this entire range, plasma osmotic concentration changing by only 250 m-osmoles/l. in the face of environmental concentration changes of 900 m-osmoles/l. Na, Cl, and K account for over 90% of plasma osmotic concentration in all external concentrations.
3. The osmoregulatory mechanisms used by the tadpoles appear to be similar to those used by euryhaline teleost fishes. In hyperosmotic media they probably drink, then excrete salts by an extrarenal pathway.
4. A metamorphosis in osmoregulatory mechanisms occurs simultaneously with morphological metamorphosis. The osmoconforming behaviour of adult frogs is fully developed in the tadpoles by developmental stage XXV. As in other anuran species there probably is no significant urea production before metamorphosis.
5. Both early embryonic development and the completion of metamorphosis beyond stage XIX apparently require low salinities.
6. The pattern of osmoregulation shown by tadpoles of the freshwater R. limnocharis is identical with osmoregulatory patterns found in adult freshwater anurans.
7. Evolutionary and ecological implications of the data are discussed.