The activity of aconitate isomerase, which is involved in the biosynthesis of trans-aconitic acid as antifeedant in brown planthoppers, was detected in Pseudomonas fluorescens LRB3W1 and Pseudomonas putida MAFF301685 but not in Pseudomonas putida MAFF301684. The enzyme activity was induced in the presence of trans-aconitate, and therefore bacteria showing the enzyme activity were easily detected by their ability to grow on the minimal medium containing trans-aconitate as the sole carbon source (AGO agar medium). Experiments on growth of plant- or insect-associated bacteria on ACO agar medium showed that most of the Gram-negative bacteria displayed the aconitate isomerase activity unlike most of the Gram-positive bacteria isolated mainly from insects. Mini-Tn5 transposon derivatives of P. fluorescens LRB3W1 lacking completely or partially their ability to grow on ACO agar medium were obtained. Southern blot analysis with a mini-Tn5 DNA probe definitely showed that the genes responsible for the biosynthesis of aconitate isomerase present on chromosomal DNA. Thus, it was suggested that genes for aconitate isomerase biosynthesis are commonly present in Gram-negative plant- or insect-associated bacteria, and also the DNA fragments including the genes were detected in P. fluorescens LRB3W1.