Salinity tolerance and preference capacities of a tubificid oligochaete Monopylephorus waltairensis was studied over a wide salinity range (F.W. to 30 %o) in vitro. This worm occurs in large numbers in the intertidal sediments of the Southern Lighter Channel at the Visakhapatnam Harbour where salinities fluctuate largely. The worms showed 50% mortality within 6 days in all salinity concentrations tested. In fresh water medium LTBO values were obtained after a period of 3 days. Also the worms preferred low salinities in all combinations except in F.W.- 2.5%o, F.W.-5%„ and F.W.-IO %„ salinity combinations. The distribution of the worm at low water mark is discussed in the light of the results obtained in the present study.