Burkholderia glumae, a bacterial pathogen on rice, produced compounds in liquid culture that, in agar diffusion assays, gave strong inhibitory action against Erwinia amylovora, the bacterium responsible for fire blight disease of apple and pear trees. Products were isolated from culture medium by cation exchange and then purified by bioassay-guided chromatographic methods. Two major products were obtained, one of which was not active when fully purified. Each product showed a single ninhydrin-staining spot on TLC and a single HPLC peak. The non-active product was deduced from NMR, MS, and chemical data, to be the tripeptide L-alanyl-L-homoserinyl-L-aspartate. The NMR data for the active product demonstrated that it contained the same tripeptide, but functionalised at the beta-carboxyl of the C-terminal aspartate, by a moiety that provided an additional 98 mass units to the parent tripeptide. Various data led to the interpretation that this moiety was a highly unusual oxygenated pyrazole structure, and thus the bioactive product was deduced to be 3-[L-alanyl-L-homoserinyl-L-aspartyl-beta-carboxy]-4-hydroxy-5-oxopyrazole. This compound was found to inhibit the growth of a number of different bacterial species.