In the soils of fields and grass lands of Colorado, mesostigmatid mites are the most abundant predators of nematodes in the below-ground food web. A new species in the assemblage of predators, Geolaelaps oreithyiae, is described. G. oreithyiae is closely related to G. aculeifer (Canestrini), but differs in size, details of ornamentation of the ventral shields, lengths of dorsal shield setae, and mode of reproduction. Virgin females of G. oreithyiae lay up to 5 eggs per day at 25°C and ali offspring develop into females, indicating a thelytokous mode of parthenogenetic reproduction. In grassland soils of the central United States, Cosmolaelaps cf vanta (Michael) (Laelapidae), Rhodacarellus silesiacus Willmann (Rhodacaridae), Amblyseius ( Neoseiulus) setulus (Fox) (Phytoseiidae), Protogamasellus mica (Athias), P. hibernicus Evans, Lasioseius youcefi Athias, L. berlesi (Ouds.) (Ascidae), and Veigaia pusilla (Berlese) (Vergaiidae) are thelytokous