The article summarizes long-term (2013–2023) monitoring results regarding the species composition and seasonal variations of fleas (Siphonaptera) on small mammals of southern Karelia. The species composition of small mammals is typical of the mid-boreal subzone and is represented by nine species, the most abundant among them being the bank vole (Myodes glareolus (Schreber, 1780)) and the common shrew (Sorex araneus Linnaeus, 1758). The flea fauna is made up of 13 species of three families: Hystrichopsyllidae (6 species), Ceratophyllidae (5), Leptopsyllidae (2). The most common species were Doratopsylla dasycnema dasycnema (Rothschild, 1897) – 44 %, Palaeopsylla soricis starki Wagner, 1930 – 18 %, Ctenophthalmus (Euctenophthalmus) uncinatus uncinatus (Wagner, 1898) – 16 %, and Peromyscopsylla silvatica (Meinert, 1896) – 9 %. The prevalence and the index of abundance of fleas on M. glareolus and S. araneus were 32 %, 0.8 and 46 %, 1.4, respectively. The dominant species of fleas on muroid rodents change in the course of the season; the prevalence and abundance index values are high in January, May, and October. Insectivores showed high infection rates in June and October, while flea numbers in the winter season declined notably. The fleas that parasitize small mammals fall into three major ecological groups: continuous breeders (A. penicilliger), warmseason species (Ct. uncinatus, P. sylvatica, M. rectangulatus, H. talpae, D. dasycnema, and P. soricis), or cold season species (R. integella, P. bidentata). In mid-boreal Karelia, there prevail the parasites whose breeding and imago life occur during the spring-to-fall period. The high abundance of fleas with this type of the annual cycle can be regarded as an adaptation to living in the North.