The paradigm of bacterial quorum sensing (QS), which mediates cell-density-dependent gene expression, usually has been studied in high-cell-density planktonic liquid cultures or in biofilms in which signal concentrations accumulate to sufficiently high levels to induce QS. Presumably under conditions with restricted diffusion of the signal molecule, smaller population sizes could achieve such a state of QS induction. The plant-pathogenic bacterium Pseudomonas syringae, in which QS controls traits involved in epiphytic fitness and virulence, occurs on leaf surfaces in aggregates of various sizes. Because leaves often harbor limited surface water, we investigated the size of aggregates that would permit QS in a nonsaturated environment. QS induction was visualized via dual fluorescence of P. syringae cells harboring a transcriptional fusion of mRFP1 with ahlI, which exhibits N-acyl homoserine lactone-dependent transcriptional activity, and a constitutive GFP marker to account for all P. syringae cells on a leaf. Confocal microscopy revealed that, on wet leaves, no QS induction was evident within 2 days after inoculation, but it increased rapidly with increasing aggregate sizes >40 and 22 cells per aggregate by 3 and 4 days, respectively. In contrast, QS induction was common in aggregates >33 cells by 2 days after inoculation on dry leaves and increased rapidly with increasing aggregate sizes >35 and 13 cells after 3 and 4 days, respectively. These observations demonstrate that small groups of cells experience QS conditions on dry leaves where signal diffusion is restricted. Quorum size of bacteria in non-water-saturated environments such as on leaves is small, and QS induction may be commonly operative.