An insertion sequence identified as a solo long terminal repeat (LTR) of a new rice copia-like retrotransposon was detected in the ORF of the Pi-b gene from the rice cv. Nipponbare, and was designated as Osr1. Osr1 consists of a 6386 bp nucleotide sequence including 965 bp LTRs on both ends with an 82% nucleotide sequence identity to the wheat Tar1 retrotransposon on reverse transcriptase. Nucleotide divergence was noted among the individual LTRs, as well as the coding region of Osr1. Various restriction fragment length polymorphism (RFLP) of LTR were detected in indica cultivars, whereas, only a few could be detected in the japonica cultivars. The population of Osr1 is lower in the wildtype rice compared with that in the domesticated cultivars. The insertion of LTR sequence in the Pi-b gene in the susceptible cultivar suggested that retrotransposon-mediated insertional mutation might play an important role in the resistance breakdown, as well as in the evolution of resistance genes in rice.