DNA transformation of forest tree species is now a striking reality and offers the possibility to generate transgenic trees with useful new characteristics. However, it is important to make the proper environmental assessment of these transgenic trees when established in field trials. For instance, the DNA released into the soil by decaying leaves and roots from the transgenic trees may become available for incorporation by soil microbes. The objective of this study was to investigate the persistence of recombinant plant marker genes in decomposing transgenic poplar leaf material. We studied the stability of the DNA encoding the neomycin phosphotransferase II resistance marker used in tree genetic engineering. DNA persistence in the environment was determined by placing transgenic poplar leaves in permeable bags that were located on weeds, on the soil, and below the soil and left under natural conditions on the site of a field trial for up to 12 months. This work is the first quantitative analysis of tree DNA stability in a natural forest environment. Our data indicate that fragments of the genetically modified DNA are not detectable in the field for more than 4 months.