Uptake of exogenous 14C-glycine betaine has been followed in the cyanobacterium Aphanothece halophytica and other species able to synthesise glycine betaine in response to osmotic stress. At 1 mmol dm−3 uptake was rapid (flux rate=29.50 nmol m−2 s−1), equilibrating at an internal concentration of 120 mmol dm−3 within 30 min. This rapid uptake, coupled with high internal accumulation, was characteristic of glycine betaine-synthesising cyanobacteria only. The 14C-glycine betaine transported was not catabolised. Kinetic studies indicated a Michaelis-Menten type relationship (K m=2.0 μmol dm−3, V max=45 nmol min−1 mm−3 cell volume), with a pH optimum of 8.0–8.5. Darkness dramatically decreased the flux rate. Higher 14C-glycine betaine levels occurred in cells growth in medium of elevated osmotic strength, and glycine betaine uptake was sensitive to changes in external salinity. A relationship between Na+ availability and glycine betaine uptake was observed, with >80 mmol dm−3 Na+ required for optimal stimulation of uptake in seawater-grown cells. Severe hyperosmotic stress (1000 mmol dm−3 NaCl) reduced the rate of glycine betaine uptake but increased internal glycine betaine concentration at equilibrium. Hypo-osmotic stress caused a decline in the internal glycine betaine concentration due to an increased rate of loss, indicating that the efflux system was also sensitive to ambient salinity changes. It is envisaged that this active transport system may be an adaptive mechanism in halophilic glycine betaine-synthesising cyanobacteria.