Fifteen days oldOreochromis niloticusjuveniles were gradually adapted to 10, 20, 30 and 35‰ sea water. The morphology of the oesophagus epithelium was observed at different intervals after transfer to higher salinities. At 10‰, no changes are evident compared to fresh water controls. Regions of single columnar epithelium, reported to play a role in ion trasport, appear within the normal pluristratified one at 20, 30 and 35‰ four weeks after transfer from fresh water. These modifications widen in individuals left for longer at high salinities.