Triclosan is a broad-spectrum antibacterial and antifungal agent1,2, which acts by previously undetermined mechanisms, that is used in products such as antiseptic soaps, toothpastes, fabrics and plastics. Here we show that triclosan blocks lipid synthesis in Escherichia coli, and that mutations in, or overexpression of, the gene fabI (which encodes enoyl reductase, involved in fatty acid synthesis) prevents this blockage. This is, to our knowledge, the first evidence that triclosan acts on a specific bacterial target, rather than as a nonspecific ‘biocide’.