The amount of trehalose in cells of the cyanobacterium Spirulina (Arthrospira) platensis increased rapidly when a high concentration of NaCl was added to the culture medium. Inhibition of sodium ion transport by amiloride and monensin significantly decreased the amount of cellular trehalose, suggesting that the influx of sodium ions into the cells is coupled with the accumulation of trehalose. The amount of maltooligosyl trehalose hydrolase (Mth) which produces trehalose from maltooligosyl trehalose increased gradually after the increase in cellular trehalose. The gene for Mth was cloned and identified by Southern blot analysis. Real time RT-PCR analysis revealed that the expression of mth was enhanced by the addition of NaCl to the culture medium. It was concluded that both catalytic activity of Mth and the synthesis of Mth protein were enhanced by the addition of NaCl to the cells.