O-Acetyl-l-Ser (OAS) is a positive regulator for the expression of sulfur (S) deficiency-inducible genes. In this study, through the isolation and analysis of Arabidopsis mutants exhibiting altered expression of S-responsive genes, we identified a thiol reductase as a regulator of the OAS levels. Ethyl methanesulfonate-mutagenized M2 seeds of transgenic Arabidopsis NOB7 carrying a chimeric S-responsive promoter driving the green fluorescent protein gene were screened for mutants with altered levels of green fluorescence compared to parental NOB7 line. One of the lines exhibited elevated levels of green fluorescence and mRNA accumulation of several endogenous S-responsive genes and carried a single recessive mutation responsible for the phenotype. OAS concentration in the rosette leaves of the mutant was about five times higher than that of wild-type plants. Based upon the high OAS levels, the mutant was named osh1-1 (OAS high accumulation). The OSH1 locus was mapped to a 30-kb region in chromosome V. DNA sequence analysis revealed no base change in this region; however, a demethylated C residue was found in the first exon of At5g01580. At5g01580 mRNA accumulation was higher in osh1-1 than in wild type, while transcript levels of other genes in the mapped region were not significantly altered in osh1-1. A line of transgenic plants overexpressing At5g01580 had elevated levels of endogenous S-responsive genes. These results suggest that elevated expression of At5g01580 is the cause of osh1 phenotype. Based on sequence similarity to animal thiol reductases, At5g01580 was tested for and exhibited thiol reductase activity. Possible roles of a thiol reductase in OAS metabolism are discussed.