The Mesostigmata is an order of Subclass Acari either parasiting to diverse invertebrates or free-living predatory in terrestrial ecosystem. Phoresy is one of the unique behaviors of wingless arthropod dispersal by attaching winged arthropods. We collected mites attached on the dorsal part of abdomen adult sciarid fly, from mushroom culture. The mites were identified as Arctoseius cetratus Sellnick, 1940 in the family of Ascidae. Arctoseius cetratus has short J2, Z1, Z2, and S1, S2 setae, not reaching the bases of following setae. Posterior half of dorsal shield with longitudinal garland of puncta between J and Z setae, peritremes extending anteriorly to midlevel of coxaⅡ. The genus Arctoseius and species Arctoseius cetratus are the first record in Korea. We provided the detail description of the species including the identification key in the family with some ecological notes.

The Mesostigmata are a great diversity of invertebrate parasite-phoretic of insects, and free-living predators in terristerial ecosystem. The Ascidae are an important group of predator that inhabit various microhabitats such as leaf litter, rotten wood, anthills and bird’s nests. Their role in those habitats is imprtant and they can be used as biological indicator and as well as biological control agent. We collected mites from sciarid fly in mushroom. The genus Arctoseius and Arctoseius cetratus were recorded for first time in Korea. We also described the habitats and identification keys to the species. Arctoseius cetratus has short J2, Z1, Z2, and S1, S2 setae, not reaching the bases of following setae. Posterior half of dorsal shield with longitudinal garland of puncta between J and Z setae, peritremes extending anteriorly to mid-level of coxaⅡ.