A polymerase chain reaction (PCR) technique was developed for detecting the presence of Xanthomonas oryzae pv. oryzae, the bacterial leaf blight (BLB) pathogen in rice seed and for studying the transmission of this bacterium from seed to plant. Primers TXT and TXT4R from an insertion sequence (IS1113) of the pathogen were used to amplify a 964-bp DNA fragment. A combined biological and enzymatic amplification (BIO-PCR) technique was used to detect the pathogen in naturally infected seed. The level of detection of TXT and TXT4R primers was 55 fg DNA of X. o. pv. oryzae, which is roughly the equivalent of seven cells (and four cells in pure culture suspension) of X. o. pv. oryzae. Hybridization of IS1113 with the amplified DNA fragment in Southern blot analysis confirmed that the 964-bp DNA fragment was amplified from X. o. pv. oryzae. The presence of the IS1113 element in strains of X. o. pv. oryzae from 16 rice-growing countries was confirmed by DNA dot blot analysis. X. o. pv. oryzae was detected from the seed washes and DNA extracted from the seed washes of naturally infected seeds of cvs Jaya and TN1. When stored at 4 degrees C, the pathogen was recovered up to 4 months and 9 months from naturally infected seeds of cvs Jaya and TN1, respectively. The BLB bacterium was also detected in seedlings, mature plants and seeds collected from plants raised from naturally infected seeds.