Large concentrations of mycosporine-like amino acids (MAAs) were found in a community of unicellular cyanobacteria inhabiting a gypsum crust developing on the bottom of a hypersaline saltern pond in Eilat, Israel. This is the first report of the occurrence of MAAs in a halophilic cyanobacterial community. Two MAAs were detected, one with an absorption maximum at 332 nm, and one at 362 nm. Intracellular MAA concentrations in the cyanobacterial community were estimated to be at least 98 mM, and this already high value is probably an underestimation. With an average molecular weight of around 300, MAAs should contribute at least 3% of the cell wet weight. While MAAs have been shown to absorb ultraviolet (UV) radiation, protecting the cells against solar radiation, they may also have an osmotic function in cyanobacterial communities inhabiting hypersaline environments, and help the cells to cope with the high salt concentrations in their environment. When the upper layer of the gypsum crust was subjected to dilution with distilled water, MAAs rapidly appeared in the outer medium, with the extent of loss of intracellular MAAs being approximately proportional to the extent of the dilution stress applied. No uptake of MAAs was observed in response to a subsequent increase in medium salinity.