The arms race between bacteria and the phages that infect them drives the continual evolution of diverse anti-phage defences. Previously described anti-phage systems have highly varied defence mechanisms(1-11); however, all mechanisms rely on protein components to mediate defence. Here we report a chemical anti-phage defence system that is widespread in Streptomyces. We show that three naturally produced molecules that insert into DNA are able to block phage replication, whereas molecules that target DNA by other mechanisms do not. Because double-stranded DNA phages are the most numerous group in the biosphere and the production of secondary metabolites by bacteria is ubiquitous(12), this mechanism of anti-phage defence probably has a major evolutionary role in shaping bacterial communities.