Landscaping of modern cities is impossible without increasing the area of green planting, which performs the function of turning the environment into more health-friendly one. Especially valuable are the species of ornamental plants that have high volatile activity. Because volatiles regulate the balance of pathogenic microflora in the air of cities, they significantly improve people's living conditions. According to the results of a previous study it was found that Thuja occidentalis L. and Th. plicata Donn ex D. Don f. “Zebrina” had the highest bactericidal activity during the winter among members of the family Cupressaceae. Given the frost, wind, gas and smoke resistance of Th. occidentalis, the aim of the study was to investigate the dynamics of phytoncide activity of the needles of this tree species during the year in a relatively clean zone and under the influence of vehicle emissions on plants in the urban ecosystem of Dnipro. The material was collected in all seasons of the year at two monitoring points in Dnipro, which differ in the level of environmental pollution by vehicle emissions. Traffic intensity was determined by the Potichuk & Pilipaka method. Antimicrobial action of Th. occidentalis was studied by the method of placing the crops of microbial test cultures under the effect of volatile secretions of needles and evaluating the result on a scale of phytoncide activity. The test subjects were Staphylococcus epidermidis, Staphylococcus aureus, Micrococcus sp., Bacillus subtilis, Esherechia coli, Pseudomonas fluorescens, Candida albicans. Studies have shown that in a relatively clean area and under the action of traffic loads, the inhibitory effect of needles on the studied strains of microorganisms changes during the year: in winter it is the smallest, in spring begins to grow, in summer is maximum, and in autumn decreases again. Under the man-made growth conditions, the greatest volatile activity of Th. оccidentalis performs on test objects such as: S. aureus, Micrococcus sp. and E. coli. When affected by car load, there is significant inhibition of phytoncide production by needles in the presence of C. albicans, to a lesser extent – Ps. fluorescens and B. subtilis. It is recommended to use Th. оccidentalis in landscaping of urban areas with moderate traffic.