This paper is the first part of a study on the oribatid mites collected from soils of the Cardoso Island, off Sao Paulo South coast. Ten species are described or recorded, namely Malaconothrus silvaticus n. sp., Epilohmannia dolosa n. sp., Teleioliodes zikani (Sell nick , 1930), Xenillus sanctipauli PerezIfiigo et Baggio, 1980, Dinoxenillus superb us Perez-Ifiigo et Baggio, 1980, Eremulus brasiliensis n. sp., Microtegeus cardosensis n. sp., Oribatella serrula n. sp., Lamellobates quadricornis n. sp. and Ceratobates jornerisae n. sp. A short account is given on Cardoso Island, pointing out its main features concerning geographic situation, climate, vegetation, etc. The study of its soil fauna is very interesting because the island conserves the primitive . vegetation , since it is a Sao Paulo State Reserve, practically untouched by man. When it has been possible a key has been introduced to make clear differences between the species concerned and the other in the genus. Drawings are presented of all the new species and of Teleioliodes zikani, that complete SELLNICK S figures.