Cells of prokaryotes (including cyanobacteria) respond to a decrease in the environmental temperature by activation of multiple genes with a low-temperature response. A decrease in temperature causes a reduction of the cell membrane fluidity, which is maintained at an optimum level due to the activity of fatty acid (FA) desaturases. We studied a temperature-dependent expression of the desB gene for the ω3-desaturase in the cyanobacterium Synechocystis, which is able to synthesize polyunsaturated FA, as well as in its double mutant (desAˉ/desDˉ), defective in the ∆12- and ∆6-desaturase genes, for which the presence of only monounsaturated FA is typical. During a decrease in temperature, in wild type cells the amount of desB mRNA increased, reaching a maximum value at 24C. In the desAˉ/desDˉ double mutant, an accumulation of the desB transcript was characterized by a maximum at 28-30C. Thus, using the desB gene encoding the ω3-FA-desaturase, it was demonstrated that a temperature-dependent expression of genes responsible for the maintenance of an optimal fluidity of cell membranes is determined by a physical state of these membranes and is regulated by a feedback mode.