The use of stable isotope techniques can assist in understanding interactions of plants with various abiotic and biotic processes. In the research, we focused on carbon (C) and nitrogen (N) isotopes because they are the most important resources influencing plant function and the biogeochemical cycles. The ¹³ C/ ¹² C and ¹⁵ N/ ¹⁴ N ratios in plants and in soils and the relationships between these ratios and biological and environmental factors of widely distributed native C ₃ plants (couch grass, plantain and yarrow) collected from two sites in St. Petersburg, Russia were studied. The soil characteristics of the sites were rather different. This had a significant effect on the isotope ratios in plants and in soils resulting in a big difference between ¹³ C/ ¹² C and between ¹⁵ N/ ¹⁴ N ratios in the soils, roots and leaves of the plants collected from the two sites. The variability of the C and N isotope ratios was also rather high among different plant species. Two main factors affected this variability: biological (plant species) and ecological (biogeochemical characteristics of soils). The ¹³ C/ ¹² C and ¹⁵ N/ ¹⁴ N ratios of roots and especially leaves were typical for a particular plant species and could differ between different plant species growing simultaneously at the same site. The soil parameters (soil texture, pH, and concentrations of total C and N in the soil) were among main factors influencing the stable isotope ratios in the soil and in different parts of a plant. A strong positive relationship between ¹³ C/ ¹² C and also between ¹⁵ N/ ¹⁴ N ratios in roots and leaves of the plants was observed. On the other hand, the correlation between ratios of ¹³ C/ ¹² C and ¹⁵ N/ ¹⁴ N calculated only for leaves or only for roots was statistically significant and negative.