Abstract We describe the occurrence in the Netherlands of three teeth of Mimomys hajnackensis (Arvicolidae, Rodentia) from three separate boreholes extending into matine deposits of the Maassluis Formation. These marine Pliocene/Early Pleistocene deposits at depth are overlain by, and interdigitate eastwards with, the freshwater deposits of the Waalre and Peize Formations (formerly known in part as the Tegelen Formation). Teeth identified as M. hajnackensis have not previously been described from the Netherlands although similar material has been reported from Pliocene deposits in Germany at Frechen and Hambach close to the Dutch border. The material described here comes from boreholes at the Polder de Biesbosch near Dordrecht, Diepenveen and Deventer. To our knowledge, these finds represent the oldest described rodent fossils from the Netherlands, corresponding to an absolute age of 2.9 to 3.3 Ma on the basis of comparison with other European localities, implying that the remains are reworked from older deposits.