Experiments were conducted to establish the effect from infestation with varying densities of Tetranychus urticae Koch on the yield of Sandra variety cucumbers grown in greenhouses. It was establish that when experimental plants had been infested with 2 mites per leaf in their 7–8 -th leaf phase during the spring, the cucumber yield was reduced by 23.8% as compared to that of the non-infested plants, while at infestation densities of 5 and 10 mites per leaf the decrease in cucumber yield was respectively 57.8 and 66.1%. During the summer period, the same infestation densities resulted in even greater lowering of the yield, 2–6% being added to the above-mentioned yield reduction percentages.