Altogether 113 nests of the tree sparrow ( Passer montanus ) from nest boxes were obtained in the Natural Reserve Šúr. In these nests we found 12 207 individuals of mesostigmatid mites (28 species). Eudominant species was Androlaelaps casalis (75%) and another 20.78% of Mesostigmata in these nests were blood-sucking mites (5 species from families Dermanyssidae and Macronyssidae). Occurence of haematophagous mite Pellonyssus reedi in Europe is reviewed. Biology and microhabitat preference of cosmopolitan nest mite Androlaelaps casalis is discussed, including its occurence in various types of birds’ nests in Slovakia .