Mercury is one of the most toxic metals to various organisms, including humans. Genes involved in mercury metabolism have been cloned from Staphylococcus aureus, and were modified here to be expressed in plants. Transgenic poplars containing both chimeric genes (p35S-merA and p35S-merB) were developed via two rounds of transformation using nos-nptII and nos-hpt genes as selectable markers. Although expression levels varied among transgenic lines, tolerance to either ionic mercury or organic mercury matched well with the degree of expression revealed by northern hybridization. In culture, these trees were tolerant to $50{mu}M$ $HgCl_2$ and $2{mu}M$ $CH_3HgCl$. Variations in mercury tolerance among the transgenic lines indicates that vigorous selection is required to select the best clones for use in phytoremediation.