During the 1997 season of Ixodes persulcatus Schulze tick activity, 579 specimens were collected by flagging in the vicinity of St. Petersburg (Morskaja, Lisy Nos). Collections were made from April to July, and at each collection time the following measurements were recorded: light intensity (in lux); temperature of soil 1–1.5 cm below leaf litter, at the soil surface, and in the air 1.5 m above the soil surface; and relative humidity. The gradient between soil at a depth of 1–1.5 cm and soil surface temperatures appeared to be the main physical parameter that initiated tick migration out of the leaf litter. At soil temperature intervals of 5–10°C and 21–26°C, ticks were found only when the soil temperature 1–1.5 cm below the soil surface was colder than at its surface. When temperature ranged from 11 to 20°C, 24% of the ticks were collected when the surface temperature was colder than that below the soil surface. Sixty percent of the ticks, especially nymphs, were collected when the soil surface and subsoil temperature difference ranged from 0 to 2.0°C. The gradient between soil temperature and relative humidity appeared to increase the importance of the gradient between the soil and surface temperature at which time the maximum number of ticks were caught. Borrelia burgdorferi sensu lato infected ticks were collected only after 1100 hours. Prior to and just after 1100 h the mean difference between soil temperatures was 4.7°C, whereas the mean difference between soil and surface temperatures was 6.4°C at this time. The most important factors that influenced the appearance of Borrelia-infected ticks were the temperature gradients that existed between the surface and soil temperatures and the soil and air relative humidity. Greater numbers of infected nymphs were collected when temperature intervals ranged from 10 to 14°C and lower numbers were collected between 15 and 20°C. No Borrelia-infected nymphs were captured when the temperature interval ranged from 21 to 26°C. Infected adults appeared to be more tolerant of higher temperatures than Borrelia-infected nymphs. A significant relationship was found between the activity of Borrelia-infected I. persulcatus and the temperature of the air, soil below the surface, at the soil surface, and relative humidity.