The species diversity of fleas (Siphonaptera) was studied in 11 species of small mammals of Karelia. The flea fauna of small mammals was found to include 14 species of 10 genera and 4 families, the species richness being the highest in 2 families – Hystrichopsyllidae (6) and Ceratophyllidae (5). The core of the fauna is made up of 5 species of fleas Doratopsylla dasycnema dasycnema, Peromyscopsylla silvatica, Palaeopsylla soricis starki, Ctenophthalmus (Euctenophthalmus) uncinatus uncinatus and Megabothris (Gebiella) rectangulatus, which have wide palaearctic distribution ranges. The bank vole and common shrew stand out in terms of the species diversity of fleas.