1. A basin approximation was used to analyse distribution patterns of different components of biodiversity (taxonomic richness, endemicity, taxonomic singularity, rarity) and conservation status of freshwater fish fauna in 27 Mediterranean Iberian rivers. 2. Basin area alone explained more than 80% of variation in native species richness. Larger basins featured not only a higher number of native species, but also more endemic and rare species and fewer diversified genera than smaller basins. 3. In contrast, smaller basins scored higher community conservation values, owing to their lower degree of invasion by introduced species. 4. The presence of dams was the most important factor determining the conservation status of fish communities, and it was also positively associated with the number of introduced species. 5. While the most important components of Iberian freshwater fish biodiversity are located in large basins, small unregulated basins feature better conserved fish communities.