Two strains of 64 endophytic bacteria, Bacillus cereus Si‐Ps1 and Pseudomonas azotoformans La‐Pot3‐3, isolated from Citrus sinensis and C. sinensis var. Thomson's leaves, respectively, reduced N‐acyl homoserine‐based quorum sensing in bioindicator strain Pseudomonas syringae pv. syringae (Pss) B728a and the biofilm production and swarming motility of field isolate Pss 3289. A homolog of aiiA gene encoding an AHL‐lactonase was found in B. cereus (Si‐Ps1), suggesting that this isolate can degrade the quorum‐sensing signal molecules of Pss 3289. The crude extract of endophytic bacterium, B. cereus (Si‐Ps1), inhibited Pss 3289 biofilm formation after 48 and 96 h by 55% and 58%, respectively. Similar reductions in biofilm formation were conferred by crude extracts of P. azotoformans (La‐Pot3‐3). Correspondingly, the number of planktonic cells in cultures treated with these extracts was higher than in control cultures, indicating a direct effect on biofilm formation and not on cell growth. In greenhouse assays, the virulence of Pss 3289 to different citrus cultivars was decreased when coinoculated with these endophytic bacteria.