The metabolic responses of the topshore cyanobacterium (blue-green alga) Rivularia atra (Roth) Born, et Flah. have been studied over the salinity range from 0 to 100% sea water. Photosynthetic competence is maintained throughout, showing only minor variation in response to salinity change. Nitrogenase activity is highest in hyposaline media (5 to 25% sea water), decreasing to approximately 55 to 60% in full-strength sea water. The principal low molecular weight, ethanol-soluble carbohydrate present within cells of R. atra is the disaccharide trehalose, occurring at concentrations up to 232 mmol kg−1(cell water) within cells incubated in 100% sea water. Trehalose concentration was found to be dependent upon the salt concentration of the bathing medium, suggesting that it played a role in the maintenance of positive turgor under conditions of varying salinity. Approximately 10 to 12% of the total intracellular trehalose pool was released upon ‘downshock’ [transfer from 100% sea water to 0% (fresh water)]. Regular release of such quantities of carbohydrate in response to ‘downshock’, e.g. on exposure to rain or freshwater at low tide, may be locally important in providing fixed carbon for associated heterotrophic organisms, although it represents a negligible input of fixed carbon to the marine littoral environment.