Abstract: The changes in the leaves of cucumber infested by spider mites after application of two biostimulants, Asahi SL and Siapton 10L, were studied as well as the development of two spider mite (Tetranychus urticae Koch) populations. The cultivation of cucumber plants cv. Aramis treated and not treated with biostimulants and their infestation with mites was conducted in the greenhouse conditions. The samples of leaves were collected for analysis after 6 weeks of experiments. The content of primary metabolites, proteins and sugars, as well as secondary metabolites, phenols, was estimated in the young and older leaves of experimental plants to calculate the ratio between them in different experimental conditions. The activity of peroxidase (POX) in the leaves of mite infested plants treated and not treated with biostimulants was also compared. The treatment of plants with biostimulants decreased phenol concentration in older leaves of both mite infested and not infested leaves. However, the young leaves of infested plants treated with Siapton SL had the highest concentration of phenols and low concentration of sugars as compared to control plants what resulted in low value of ratio sugars: phenols. High increase in peroxidase activity was found in all plants infested by spider mites. An application of both biostimulants on cucumber plants caused, however, significant increase in peroxidase activity as compared to not treated plants. The spider mite populations in all plants treated with biostimulants were lower than on control ones.