The regulation of the expression of thrS, the structural gene for threonyl-tRNA synthetase, was studied using several thrS-lac fusions cloned in lambda and integrated as single copies at att lambda. It is first shown that the level of beta-galactosidase synthesized from a thrS-lac protein fusion is increased when the chromosomal copy of thrS is mutated. It is also shown that the level of beta-galactosidase synthesized from the same protein fusion is decreased if wild-type threonyl-tRNA synthetase is overproduced from a thrS-carrying plasmid. These results strongly indicate that threonyl-tRNA synthetase controls the expression of its own gene. Consistent with this hypothesis it is shown that some thrS mutants overproduce a modified form of threonyl-tRNA synthetase. When the thrS-lac protein fusion is replaced by several types of thrS-lac operon fusions no effect of the chromosomal thrS allele on beta-galactosidase synthesis is observed. It is also shown that beta-galactosidase synthesis from a promoter-proximal thrS-lac operon fusion is not repressed by threonyl-tRNA synthetase overproduction. The fact that regulation is seen with a thrS-lac protein fusion and not with operon fusions indicates that thrS expression is autoregulated at the translational level. This is confirmed by hybridization experiments which show that under conditions where beta-galactosidase synthesis from a thrS-lac protein fusion is derepressed three- to fivefold, lac messenger RNA is only slightly increased.