Altitudinal changes in collembolan communities were studied by sampling soil microarthropods along a gradient from 950 to 2150 m a.s.l., under a wide range of forest vegetation types. A multivariate method showed that most changes in species composition followed changes in soil chemistry, humus forms and vegetation. A transition from mull to mor humus, with concomitant soil acidification, was observed with increasing elevation. It was observed that at a given elevation, changes in soil acidity occurring in the course of forest dynamics exerted the same effects than altitude, thus soil acidity explained better the composition of collembolan communities. Densities and local diversity of Collembola were observed to increase with soil acidity, which can be explained by (i) physiological adaptations to acid soils inherited from palaeozoic times and (ii) more habitat and food resources when organic matter accumulates at the top of the soil profile.