The root structure of a mature 12-year-old mulberry tree growing in a former waste disposal basin and the relationship of the tree's root system to the distribution of polycyclic aromatic hydrocarbons (PAHs) in soil/sludge beneath the tree were examined. Root excavation showed that large roots were restricted to the upper 60 cm of the root zone, whereas medium-sized (0.3 to 2.5 cm) and fine (<0.3 cm) roots were present throughout the root zone reaching depths of 110 cm. Comparison of the concentrations of PAHs in the root zone with those in the sludge beneath the root zone (>2.0 m) showed dramatic differences. In the upper 60 cm of the root zone, where roots have prevailed for potentially 12 years, the PAH concentrations were approximately 10 to 20% of those in the sludge. At lower rooting depths 60 to 100 cm, where fewer roots have been present for a shorter time period, the PAH concentrations have decreased but to a lesser degree. These field results provide evidence that the mulberry root system develops gradually over time in a vertical direction with fine roots serving as the leading edge of a moving front. The degradative properties of this biological front (root-microbe combination) toward toxic chemicals hold great importance for successful development of phytoremediation technology designed to degrade deep contaminants in former sludge basins.