Regulation of cytosine methylation in the plant genome is of pivotal in determining the epigenetic states of chromosome regions. Relative tolerance of plant to deficiency in cytosine methylation provides unparalleled opportunities to study the mechanism for regulation of cytosine methylation. The Decrease in DNA Methylation 1 (DDM1) of Arabidopsis thaliana is one of the best characterized plant epigenetic regulators that are necessary for maintenance of cytosine methylation in genomic DNA. Although cytosine methylation could affect various aspects of plant growth and development including those related to agricultural importance, orthologs of DDM1 in plants other than Arabidopsis has not been studied in detail. In this study, we identified two rice genes with similarity to Arabidopsis DDM1 and designated them OsDDM1a and OsDDM1b. Both of the rice DDM1 homologs are transcribed during development and their amino acid sequences are 93 % identical to each other. Transgenic rice lines expressing the OsDDM1a cDNA in the antisense orientation exhibited genomic DNA hypomethylation. In those lines, repeated sequences were more severely affected than a single copy sequence as is the case in Arabidopsis ddm1 mutants. Transcripts derived from endogenous transposon-related loci were up-regulated in the antisense OsDDM1 lines, opening a possibility to identify and utilize potentially active transposons for rice functional genomics.