The present work is a report on the salinity tolerance of the lancelet Branchiostoma lanceolatum of the Madras waters. Lancelets were maintained in glass troughs (30.5*15.0 cm) under about 5 cm of sea-water of salinity 34%_o, Samples of 10 lancelets of the same size were transferred to media of low salinity after an acclimation period of 15 days in sea water of 34 %_o. It was noted that on direct and sudden transfer there was one hundred per cent mortality of lancelets in sea-water of 10 %o and 17%o; 50% mortality in 26%o and nil morality in 29%o. Further, survivals possible upto 14%o only, when the lancelets taken through a graded series of sea water of above mentioned salinity with about 48 hrs of acclimation in each medium. Results are discussed with refence to the ability of amphioxus to invade estuarine waters, and the general flexibility of osmoregulation in marine lancelets is stressed.