In a preliminary screening, the methanol extract of Cinnamomum tamala leaves was found to inhibit the swarming motility of Pseudomonas aeruginosa. Bioassay-guided fractionation by silica gel column chromatography led to the identification of cinnamtannin B1 (1) as one of the active components of the extract. It inhibited the swarming motility (at 12.5 mu g/mL) and biofilm formation (at 25 mu g/mL) ofP. aeruginosa. Comparative gene expression analysis revealed downregulation of rhlA and fliC genes upon treatment with the tannin. The tannin may be affecting rhamnolipid and flagellin production. Thus, cinnamtannin B1 is an active component of C. tamala responsible for inhibiting the swarming motility of p. aeruginosa.