In plants, transgenes can be silenced at both the transcriptional  and post-transcriptional levels . Methylation of the transgene promoter correlates with transcriptional gene silencing (TGS)  whereas methylation of the coding sequence is associated with post-transcriptional gene silencing (PTGS) . In animals, TGS requires methylation and changes in chromatin conformation . The involvement of methylation during PTGS in plants is unclear and organisms with non-methylated genomes such as Caenorhabditis elegans or Drosophila can display RNA interference (RNAi), a silencing process mechanistically related to PTGS . Here, we crossed Arabidopsis mutants impaired in a SWI2/SNF2 chromatin component (ddm1 ) or in the major DNA methyltransferase (met1  and E. Richards, personal communication) with transgenic lines in which a reporter consisting of the cauliflower mosaic virus 35S promoter fused to the beta-glucuronidase (GUS) gene (35S-GUS) was silenced by TGS or PTGS. We observed an efficient release of 35S-GUS TGS by both the ddm1 and met1 mutations and stochastic release of 35S-GUS PTGS by these two mutations during development. These results show that DNA methylation and chromatin structure are common regulators of TGS and PTGS.