The aim of the paper is detect diversity of species, and seasonal dynamics of one group of ectoparasites – fleas (Siphonaptera) found in the fur of small mammals, living in areas of the Podunajska lowland, Hron upland and Nitra upland. The trapping of small mammals was done in years 2016 to 2020 in 35 localities of these three areas. The research is mainly related to the shift of the borders of the occurrence of the Striped field mouse host from the southern parts of Slovakia towards the northwest. Captured and ectoparasitologically examined was 5332 small mammals – 17 host species in total. Of these, 572 individuals (10.7 %) were infested (flea positive). From their fur, we got 1268 ectoparasites of 9 species, namely Hystrichopsylla orientalis, Ctenophthalmus agyrtes, Ctenophthalmus assimilis, Ctenophthalmus solutus, Doratopsylla dasycnema, Palaeopsylla soricis, Megabothris turbidus, Megabothris walkeri and Nosopsyllus fasciatus.