A selenium-containing tRNA from C. sticklandii was isolated and identified as a glutamate accepting tRNA. There is one atom of selenium per active tRNA-Glu molecule. The presence of the selenonucleoside (mam-5-Se-2-U) in this tRNA-Glu is essential for its acylation with glutamate. Preliminary results indicate that seleno-tRNAs may also exist in mammalian cells.