The widespread use of antibiotics has led to the emergence of antibiotic resistance, which poses a significant challenge to the treatment of bacterial infections. While the high concentrations of antibiotics used therapeutically can select for resistant mutants, the role of much lower antibiotic concentrations in natural environments remains unclear. This study demonstrates that resistant bacteria can be selected at extremely low antibiotic concentrations, even when present at very low initial fractions. Using highly sensitive competition experiments, the researchers found that for three clinically important antibiotics, drug concentrations several hundred times below the minimal inhibitory concentration (MIC) of susceptible bacteria could enrich for resistant bacteria. Additionally, *de novo* mutants can be selected at sub-MIC concentrations. The study also provides a mathematical model predicting how rapidly such mutants would take over in a susceptible population. These findings suggest that low antibiotic concentrations in natural environments are important for the enrichment and maintenance of resistance in bacterial populations, highlighting the need for measures to reduce antibiotic pollution.The widespread use of antibiotics has led to the emergence of antibiotic resistance, which poses a significant challenge to the treatment of bacterial infections. While the high concentrations of antibiotics used therapeutically can select for resistant mutants, the role of much lower antibiotic concentrations in natural environments remains unclear. This study demonstrates that resistant bacteria can be selected at extremely low antibiotic concentrations, even when present at very low initial fractions. Using highly sensitive competition experiments, the researchers found that for three clinically important antibiotics, drug concentrations several hundred times below the minimal inhibitory concentration (MIC) of susceptible bacteria could enrich for resistant bacteria. Additionally, *de novo* mutants can be selected at sub-MIC concentrations. The study also provides a mathematical model predicting how rapidly such mutants would take over in a susceptible population. These findings suggest that low antibiotic concentrations in natural environments are important for the enrichment and maintenance of resistance in bacterial populations, highlighting the need for measures to reduce antibiotic pollution.