Abstract: In the present work, generation of hydrogen peroxide in Chlamydomonas reinhardtii Dang. cells under hypothermia was demonstrated. Analysis in these cells of genes' transcription encoding the enzymes involved in detoxification of H₂O₂ in them, such as ascorbate peroxidases (APX1, APX2 and APX3), catalases (CAT1 and CAT2), and glutathione peroxidases (GPX1, GPX3, GPX4 and GPX5), showed that action of low temperature (7°C) leads to lowering transcription of genes encoding catalases and ascorbate peroxidases. At the same time, induction of three GPX genes, GPX1, GPX3, and GPX5, was found under these conditions. Maximal (50 times) elevation in transcription levels was determined for the GPX3 gene. As a result, the gene encoding glutathione peroxidase 3 is suggested as a potential marker for C. reinhardtii’s response to cold treatment. In addition, it was shown that hydrogen peroxide is involved in the regulation of the induction of the GPX3 gene under the action of low temperature.