Eighteen flea species of four genera associated with bats (Chiroptera: Vespertilionidae, Rhinolophidae, Miniopteridae, Molossidae) are registered in Russia, based on a literature review as well as our own data. They are as follows: Myodopsylla trisellis, Ischnopsyllus (Ischnopsyllus) dolosus, Ischnopsyllus (I.) elongatus, Ischnopsyllus (I.) intermedius, Ischnopsyllus (I.) plumatus, Ischnopsyllus (I.) needhami, Ischnopsyllus (I.) obscurus, Ischnopsyllus (I.) octactenus, Ischnopsyllus (I.) simplex, Ischnopsyllus (I.) variabilis, Ischnopsyllus (Hexactenopsylla) hexactenus, Ischnopsyllus (H.) petropolitanus, Ischnopsyllus (H.) ussuriensis, Ischnopsyllus (H.) comans, Nycteridopsylla pentactena, Nycteridopsylla longiceps, Nycteridopsylla eusarca, Rhinolophopsylla unipectinata unipectinata. Ischnopsyllus (I.) elongatus and Ischnopsyllus (I.) octactenus were recorded in Kaluga Region and Vladimir Region for the first time. Ischnopsyllus (I.) variabilis in Kaluga Region is a newly documented find. Studied bat flea fauna exhibits a distinctly Western Palearctic appearance.

Mercury is considered to be one of the chemical elements posing the greatest threats to the health of most animals and can be transferred from aquatic ecosystems to terrestrial food webs. Many bat species forage above water, and their food sources include aquatic and amphibious organisms. Bats are very sensitive to the slightest changes in the environment. The objective was to determine the accumulation of mercury in the fur of insectivorous bats in summer habitats in an area with limited anthropogenic activity in the conditions of the middle taiga in the northwest European part of the Russian Federation. In the studied species, the average values of the metal’s content (μg/g) increased in the following order: Myotis daubentonii (3.294 ± 0.934), Myotis dasycneme (3.909 ± 0.543), Vespertilio murinus (8.011 ± 1.136), Pipistrellus nathusii (8.366 ± 0.546), and Nyctalus noctula (8.408 ± 1.386). The key factor regarding the mercury accumulation in each bat species is the foraging strategy. The mercury content in the fur of adult bats was higher than in subadults.