Fungus-growing ants and their microbial symbionts have emerged as a model system for understanding antibiotic deployment in an ecological context. Here we establish that bacterial symbionts of the ant Trachymyrmex septentrionalis antagonize their most likely competitors, other strains of ant-associated bacteria, using the thiopeptide antibiotic GE37468. Genomic analysis suggests that these symbionts acquired the GE37468 gene cluster from soil bacteria. This antibiotic, with known activity against human pathogens, was previously identified in a biochemical screen but had no known ecological role. GE37468's host-associated defense role in this insect niche intriguingly parallels the function of similar thiopeptides in the human microbiome.