There is no large difference between the lower lethal salinity for the veliger larva and the adult of Nassarius obsoletus. The region of stress in snails is between 12.5%o andl3.5%o; that of the early larva is between 14.0andl5.5%0. Throughout larval development no change occurred in the value of the lower lethal salinity. Differences in growth rate of N. obsoletus larvae observed at salinities about 24 are usually slight. However, at 21%oand less there is a statistically significant drop in growth rate, while at mean salinity of 17.7%o, it was not possible to rear the larvae to the completion of development and metamorphosis. The maximum inhibition of growth attributable to the affects of salinity, within the range at which development of the larvae is completed, is between approximately 13% and 40% and averages about 20%. This is less than that of other ecological factors known to retard growth. The net result of reduced salinity, within the lower third of the range at which the larval development of N. obsoletus is completed, is an increase in the length of time to reach the creeping-swimming stage which precedes metamorphosis, and an increased mortality of larvae as the limit of salinity tolerance is reached. A comparison of data from N. obsoletus with that of another molluscan species, the pelecypod Venus mercenaria, which is found at approximately the same salinities within Atlantic coast estuaries along the United States, shows striking similarities.