The study reported here stems from experiments performed upon cirripedes and bivalve molluscs exposed to fluctuating sea water concentrations. It is obvious that these two groups of benthic organisms are capable of shutting themselves off to a greater or lesser degree from the external environment by opercular or shell valve closure, thus avoiding the osmotic stresses associated with exposure to lowered salinity levels. With the development by Davenport, Gruffydd & Beaumont (1975) of an apparatus to deliver fluctuating salinity regimes to experimental animals in a repeatable, routine manner, it has become possible to establish the sea-water concentrations at which such mechanisms operate.