Covalent modification of proteins by phosphate transfer reactions constitutes a major mechanism of regulation in higher eukaryotes. Recently, phosphorylation of eukaryotic elongation factors has been described. Analysis of Escherichia coli proteins revealed several of them to be phosphorylated. Various lines of evidence lead us to conclude that one of these proteins is identical to elongation factor (EF) Tu, which can be phosphorylated in vivo at one of its threonine residues. Structural analysis showed that one fragment of the phosphorylated EF-Tu is highly resistant to tryptic digestion. Phosphorylation of eubacterial EF-Tu is not restricted to the E. coli factor but could also be demonstrated for Thermus thermophilus HB8 EF-Tu. Overexpression of tufA did not increase the number of EF-Tu molecules to be phosphorylated. This may indicate that a constant but limited amount of EF-Tu is modified, possibly for a specific function. Phosphorylation of EF-Tu could also be demonstrated in vitro. Upon analysis of subcellular fractions the highest kinase activity was found in the ribosomal fraction of E. coli. Protein sequencing of both the in vivo and in vitro phosphorylated protein revealed position 382 as the modified threonine residue.