The ontogeny of salinity tolerance was studied in the tilapias Oreochromis aureus, O. niloticus, and an O. mossambicus x O. niloticus (M x N) hybrid, spawned and reared in freshwater. Several indices were employed as practical measures of salinity tolerance: (1) median lethal salinity-96 hours, defined as the salinity at which survival falls to 50% 96 hours after transfer from freshwater to varying salinities; (2) mean survival time, defined as the mean survival time over a 96-hour period following direct transfer from freshwater to seawater (32 ppt); and (3) median survival time, defined as the time at which survival falls to 50% following direct transfer from freshwater to seawater. The ontogenetic changes observed in salinity tolerance were determined to be more closely related to body size than to chronological age. No consistent relationship was observed between salinity tolerance and condition factor. Results confirm earlier research indicating that O. mossambicus and O. niloticus hybrids have a superior salinity tolerance. Combination of hybridization with a maximization of early freshwater growth through temperature control and hormone application should optimize conditions for economical culture of tilapia in brackish water or seawater. (Author abstract, modified).