Abstract The growth of the mangrove red alga Bostrychia simpliciuscula Harvey ex J. Agardh was studied in relation to a gradient of salinities (5–70 ppt) and photon fluences rates (2.5–60 μmol m −2 s −1). The photosynthesis-irradiance ( PI) curve and the effect of salinity on the intracellular d-sorbitol and d-dulcitol concentrations were also examined. Bostrychia simpliciuscula grew under all salinities tested with a preference for brackish water conditions. The growth versus light pattern and the PI curve indicate an adaptation to shaded habitats. This alga synthesised and accumulated d-sorbitol in parallel with increasing salinities over the whole range of 5–60 ppt, while d-dulcitol was only involved in the osmotic acclimation between 15 and 45 ppt. The abundance of B. simpliciuscula under the extreme environmental conditions predominant in mangrove swamps of southeast Asia can be explained by these physiological properties.