The halotolerant phytoplankton alga Dunaliella tertiolecta was found to be able to synthesize considerable amounts of sucrose. Because this is in contradiction to the current literature, the physiologic conditions required for sucrose biosynthesis were investigated. Sucrose is only synthesized above a jump temperature, which depends on the NaCl concentration in the medium. Unlike earlier results, which showed that glycerol is synthesized only under photosynthetic conditions, sucrose is also produced in the dark, however at a reduced rate. In the dark, sucrose synthesis ceases after 24 h, thus indicating that sucrose is formed from storage substances.