Biological activity in the urban modern and medieval soils of St. Petersburg was determined using soil samples taken from sections located at the historical center of this city nearby the Kazan Cathedral, the Twelve Colleges building (now the main building of St. Petersburg State University), and on the site where the Swedish fortress Nienshants formerly existed. The studied parameters of biological activity included the microbial transformation rate of organic matter under aerobic and anaerobic conditions, the intensity of denitrification and nitrogen fixation, and the amount of microbial biomass. This investigation is the first attempt to comparatively study modern urban anthropogenically impacted soils and buried soils that had formed the soil cover of this region before St. Petersburg was founded. The major microbiological and physicochemical parameters of the soils were subjected to correlation analysis.