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Although the soil is a major reservoir of biodiversity, our knowledge ofits mesofauna remains scanty, especially in the tropics. The diversity oforibatids (149 adult oribatid mite species) is analyzed for the first time in anAfrican soil and studied in three ecosystems of a regressive sere: forest,woodland and savanna. Savanna is the richest ecosystem overall, with 105collected species, whereas the mean number of species per releve (αdiversity) is highest in forest. In barren soils, the number of species observedalong the sere drops regularly from the typical forest to the savanna. However,this pattern is complicated by other factors acting at different scales. Theincrease of oribatid richness parallels that of habitat complexity, from barrensoil to termitaria colonized by grasses and trees. On a finer scale, soilproperties also influence species richness, either indirectly through density(water content) or directly (total nitrogen, C/N ratio, organic matter), buttheir importance varies in relation to seasons. Most exclusive species (nearly90%) are housed in the two extreme types of vegetation, forest and savanna. On afiner scale, two habitats, the typical forest and the termitaria in the savanna,are remarkable by the number of exclusive species and are worth protectingthrough effective conservation measures.