The spread of antibiotic resistance is a major public health concern. Czekalski et al. investigate whether ozonation of wastewater can help to combat this spread by eliminating resistant bacteria. In laboratory experiments, ozone doses that can be used in full-scale applications disrupted intracellular resistance genes. However, ozonation of secondary effluent at a wastewater treatment plant did not affect the abundance of intracellular resistance genes, and multiresistant bacteria partly regrew after ozonation. The results have important implications for wastewater treatment plants that are planning to implement ozonation.