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Abstract: The effects of spatial scale on the assessment of soil biodiversity were investigated through a field study conducted in the Pelagian Islands, Sicilian Channel, southern Mediterranean. The oribatid mite community in this archipelago was investigated from both an ecological and a biogeographical point of view. The following hierarchically nested spatial scales were considered: 1—archipelago (γ-diversity); 2—island (α- and β-diversity between the islands); 3—sites (α- and β-diversity within each island). Quantitative and semi-quantitative soil sample replicates were collected during the wet season (autumn, 1999) in Lampedusa and Linosa, the two major islands of the archipelago. Data revealed that the sample α-diversity of the two islands is similar. The density of species followed a geometrical trend (a few dominant species with the remainder fairly uncommon) typical of communities in which a single environmental factor dominates species ecology. Community structure differed significantly between the two islands. Species turnover (β-diversity) was very high at every spatial scale, from the sites to the archipelago. Biogeographical results highlighted great differences between faunas in the two islands. The formulation of hypotheses for biodiversity patterns is strongly scale dependent: the heuristic and conservation value of the biogeographical approach increases from local to regional scales, because the importance of historical factors increases while that of ecological factors decreases. [Copyright &y& Elsevier]