Cyclic nucleotides are important second messengers, and for many (such as cyclic di-GMP) their roles in a diverse range of bacterial developmental processes and functions, including virulence, are well known. By contrast, the role of cyclic GMP (cGMP) as a bacterial signalling molecule is largely unknown. Ryan and colleagues screened transposon mutants of Xanthomonas campestris and identified two strains with reduced cGMP synthesis, both of which had defects in XC_0250, a gene encoding a guanylyl cyclase that converts GTP to cGMP. Compared with the wild-type strain, an XC_0250 deletion mutant showed reduced biofilm formation and virulence in plants, which was reflected by altered transcription of genes known to be involved in these processes. Deletion of XC_0249, a gene with a putative cyclic-mononucleotide-binding domain and a diguanylyl cyclase domain, had similar phenotypic and transcriptional effects. On the basis of further structural and functional data, the authors propose a model that directly links cyclic mononucleotide and dinucleotide signalling, in which cGMP induces XC_0249-mediated cyclic di-GMP synthesis.