Dunaliella salina and D. bardawil are well-known microalgae accumulating high levels of b-carotene under growth-limiting conditions. In both taxa, this pigment is primarily composed of the isomers 9-cis and all-trans. The 9-cis b-carotene occurs only in natural sources and is the most attractive from a commercial point of view. The conditions that enhance the preferred accumulation of 9-cis b-carotene in D. salina are controversial and they have not been well established yet. This study examined the effect of salinity on the quantity and quality of total carotenoids and b-carotene isomers accumulated by D. salina (strain CONC-007) and D. bardawil (strain ATCC 30861) grown in two media with different nutritional compositions (PES and ART) and at salt concentrations of 1M, 2M and 3M NaCl. Total carotenoids were determined by spectrophotometry and b-carotene isomers, by HPLC. The highest carotenoid contents per cell were obtained at 2M NaCl in both taxa. In both media, an increase of the 9-cis/all-trans b-carotene ratio was observed in D. bardawil when the salt concentration increased, with a maximum value of 2.6 (in ART medium at 3M NaCl). In D. salina this ratio did not exhibit the same pattern, and the salt concentrations for maximal ratios were different in both media. The highest ratio obtained for this strain was 4.3 (in ART medium at 2M NaCl).