An experimental salinity gradient system has been developed to meet a need in efforts to determine the salinity preference of small benthic and estuarine organisms. In the described apparatus a slanting tank provides a homogeneous bottom for benthic organisms throughout the full range of test salinity levels. Continous gradients produced in this apparatus are shown to be quite stable for several days. They also differ from earlier systems in lacking either physical barriers, salinity discontinuities, or water currents. The apparatus can also be used to determine salinity preferences of small free-swimming organisms.