A selectable marker gene facilitates the detection of genetically modified plant cells during transformation experiments. So far, these marker genes are almost exclusively of two types, conferring either antibiotic resistance or herbicide tolerance. However, more selectable markers must be developed as additional transgenic traits continue to be incorporated into transgenic plants. Here, we used mercury resistance, conferred by the organomercurial lyase gene, as a selectable marker for transformation. The merB gene from Streptococcus aureus was modified for plant expression and transferred to a hybrid poplar (Populus alba$ imes$Populus glandulosa), using the stem segment-agrobacteria cocultivatiion method. The transformed cells were selected on a callus-inducing medium containing as little as $1 {mu}M$ methylmercury. Subsequent plant regeneration was done in the presence of methylmercury. Resistance to Hg was stably maintained in mature plants after two years of growth in the nursery. We suggest that this gene could serve as an excellent selectable marker for plant transformation.