We report the identification of fatty acids as mediators of intercellular signalling in Pseudomonas putida, and between Pseudomonas aeruginosa and P. putida. Tetradecanoic acid and fatty acids of similar chain length are present in supernatants of these strains and activate population density-dependent expression of ddcA, a gene involved in corn seed and root colonization by P. putida KT2440. Consistently, significant amounts of these compounds were also found in corn root exudates. The signalling pathway involves the two-component regulatory system formed by RoxS and RoxR, which had been previously shown to control expression of ddcA and of a set of genes related to the redox balance of P. putida cells. Production of the fatty acid signal in P. aeruginosa is under the control of the LasI/LasR and RhlI/RhlR quorum sensing systems. Our data indicate that in terms of cell–cell communication, P. putida KT2440 employs mechanisms closer to those of plant pathogens such as Xanthomonas spp. and fungi like Candida, which also rely on fatty acid derivatives.