For the purpose of investigating the relation between the oribatid fauna and the vegetation of ground, one year soil sampling test (1959/60) was achieved in a west suburb of Tokyo. Two adjacent groves of different vegetation, i.e. Quercus acutissima CARRUTH. and Pinus densiflora SIEB. et ZUCC., were selected. The soil sampling was made with a can of 8cm in diameter and 5cm in depth. Thirty four soil samples, 2 samples per each sampling of 17 days, were taken out from each grove between 9.30a.m. and 10.30a.m. and were dried in improved BERLESE apparatus to collect oribatid mites.<br>Of a total of 3874 mites (32 species), 2165 (30) originate from Quercus-grove and 1709 (21) from Pinus-grove. It tells that the soil under Quercus-grove have a fauna of oribatid mites richer than that of Pinus-grove. The species peculiar to Quercus-grove are Meristacarus sp., Nanhermannia parallela AOKI, Belba sp., Gymnodamaeus sp., Oppia quadricarinata (MICH.), Oppia sp. 1, Oppia sp. 3, Tetracondyla clavata AOKI, Anachipteria grandis AOKI, Ceratozetes japonicus AOKI and Phthiracalurus rostralis WILLM. The species peculiar to Pinus-grove are Eremaeus hepaticus (KOCH) and Eremaeus sp. The species obtained from both groves are Epilohmannia ovata AOKI, Cryptacarus hirsutus AOKI, Eohypochthonius gracilis (JACOT), Liochthonius ensifer (STR.), Nothrus biciliatus KOCH, Eremobelba japonica AOKI, Suctobelba naginata AOKI, Oppia nova (OUDMS.), Oppia sp. 2, Oppia sp. 4, Scheloribates laevigatus (KOCH), Tectocepheus velatus (MICH.), Cultroribula latus AOKI, Punctoribates punctum (KOCH), Protoribates lophotrichus (BERL.), Protoribates sp., Galumna sp., Rhysotritia ardua (KOCH), and Hoplophorella cucullata (EWING). Whereas the Pinus-grove contains only two peculiar species and almost all the species from this grove are common to the Quercus-grove, the latter has eleven peculiar species.<br>From a point of view of dominancy and constancy, the species found from the two groves were classified according to the standard offered by KROGERUS (1932). Anachipheria grandis AOKI and Suctobelba naginata AOKI were qualified for dominant species in Quercus-grove, Protoribates lophotrichus (BERL.), Epilohmannia cylindrica (BERL.) Punctoribates punctum (KOCH) and Oppia nova (OUDMS.) were dominant in Pinus-grove, and Eohypochthonius gracilis (JACOT), Cultroribula latus AOKI and Rhysotritia ardua (KOCH) were dominant in both groves. The constant species for both Quercus- and Pinus-groves were Eohypochthonius gracilis (JACOT), Cultroribula latus AOKI, Epilohmannia cylindrica (BERL.), Rhysotritia ardua (KOCH), Protoribates lophotrichus (BERL.) and Oppia nova (OUDMS.). In addition, Anachipteria grandis AOKI, Epilohmannia ovata AOKI, Cryptacarus hirsutus AOKI, Ceratozetes japonicus AOKI and Scheloribates laevigatus (KOCH) were recognized as constant species exclusively for _Quercus-grove. However, none of species was found as constant species exclusively for Pinus-grove. Considering their dominancy, constancy and commonness between both groves a determination of the characteristic species in each grove was made: Anachipteria grandis AOKI and Suctobelba naginata AOKI belong to Quercus-grove, and Epilohmannia cylindrica (BERL.), Protoribates lophotrichus (BERL.) and Oppia nova (OUDMS.) to Pinus-grove as characteristic species. It is obvious that such a difference in the oribatid fauna between the two groves results only from the difference of vegetation