The mineral weathering ability of 45 bacterial strains belonging to the genus Collimonas and coming from various terrestrial environments was compared to that of 5 representatives from the closely related genera Herbaspirillum and Janthinobacterium. Using glucose as the sole carbon source in a microplate assay for quantifying the release of iron and protons from biotite, all Collimonas strains proved to be very efficient weathering agents, in contrast to the Herbaspirillum and Janthinobacterium strains. The weathering phenotype was also evident during growth of collimonads on mannitol and trehalose, but not on gluconic acid. All Collimonas strains were able to solubilize inorganic phosphorus and produce gluconic acid from glucose, suggesting that acidification is one of the main mechanisms used by these bacteria for mineral weathering. The production of siderophores may also be involved, but this trait, measured as the ability of collimonads to mobilize iron, was shared with Herbaspirillum and Janthinobacterium strains. These findings are discussed in an ecological context that recognizes collimonads as mycophagous (fungal-eating) and efficient mineral weathering bacteria and suggests that this ability has evolved as an adaptation to nutrient-poor conditions, possibly as part of a mutualistic relationship with mycorrhizal fungi.