The oxidation of pyrite in a mine tailings heap in Romania was studied to clarify the contribution of acid drainage to the death of vegetation on the eastern slope of the tailings. Where vegetation died, pyrite oxidation was detected as deep as 1 m, as indicated by the brownish color of the samples. At these sitesThiobacillus ferrooxidans-like bacteria were present with cell counts of 10 3g andThiobacillus thiooxidans-like bacteria occurred with cell counts of 10 g. At the western slope, where vegetation thrived, cell counts of < 10 g were measured. Correspondingly, leaching activity and concentrations of pyrite oxidation products such as sulfate and elemental sulfur were markedly higher on the eastern slope than on the western slope. These differences were mainly a result of the high acid neutralization potential in the material from the western slope, which kept the pH in a neutral range allowing for plant growth. In contrast, on the eastern slope with its low acid neutralization potential, the pH dropped to 3 to 4, increasing the solubility of phytotoxic elements. The death of the vegetation could result from the lowered pH and the increased amount of toxic elements, or, most likely, a combination of these factors.