The U.S. Bureau of Mines is conducting research on the removal of heavy metal ions from residual waters. This research has included exploratory batch and small-scale continuous laboratory studies on the biohydrometallurgical removal of selenium from agricultural drainage waters. Tests were conducted on alkaline drainage waters obtained from the Kesterson National Wildlife Refuge, San Joaquin Valley, California. The water contained 280 to 3,800 ppb selenium present as seo4 2-. Using indigenous bacteria, the selenium was reduced from selenate, se(VI), to selenite, se(IV), and then precipitated in elemental form from solution. Selenium removal ranged from 87 to 96 pct, regardless of the initial selenium concentration; in one system, the selenium level was reduced from 3,800 ppb to less than 150 ppb in about 1 h. The technique was effective when the bacteria were added directly to the water, or when the water was pumped through a substrate that had been previously inoculated with bacteria.