Biodiversity is a complex issue, frequently simplified by equating it with species richness. In this paper an assessment of river systems biodiversity in southern Spain is carried out. Four systems here analysed run through mountains of medium altitude and they are remarkable for their acceptable species richness, high proportion of spp. with a restricted western Mediterranean distribution and low ratio of spp. per genus. Species distinctness was estimated according the schemes of R.I. VANE-WRIGHT et al. (1991, Biol. Conserv. 55: 235-254) and R.M. MAY (1990, Nature, Land. 347: 129-130), based on the information content of the topology of a hierarchical classification.