The rice blast fungus Magnaporthe oryzae is an extremely effective plant pathogen that produces specialized infectious structures known as appressoria. Four hundred thirty eight endophytic bacterial strains were isolated from trees growing in Vietnam and screened for antifungal activity. A strain of a Gram-negative bacterium, Stenotrophomonas maltophilia was found to have an activity that inhibits appressorium formation of M. oryzae. Activity-guided fractionation of the bacterial extract yielded a hydrophobic active fraction, which was purified through MPLC and PTLC, and analyzed by GC-MS. Three kinds of fatty acid, 13-methyltetradecanoic acid, 12-methyltetradecanoic acid (12-MTA), and palmitic acid were identified from the active fraction, among which, only 12-MTA had the activity. It specifically inhibited the appressorium formation with $IC_{50}$ value of $83.5;{mu}M$. The action mechanism and agrochemical use of 12-MTA should be further studied.