Abstract: to investigate whether rice adult plant resistance (APR) is related to activation of long terminal repeat (LTR) retroelements, polymorphism of LTR retroelemnts Tos17 are studied using SSAP (Sequence-Specific Amplification Polymorphism) in seedlings and adult plants of rice cultivar Wase Aikoku 3 inoculated with Xanthomonas oryzae pv. Oryzae, water and those untreated. Of 2000 fragments detected, 9 and 2 are activated in seedlings and adult plants induced by development, respectively; each 3 fragments are activated in seedlings and adult plants induced by pathogen, respectively. The number of activated Tos17 induced by development is more in seedlings than that in adult plants while it is equal due to pathogen induction in seedlings and adult plants, suggest that the activation of Tos17 induced by development probably associate with rice APR to bacterial blight.