A negative regulatory gene, soxR, was found upstream of the sarcosine oxidase gene, soxA, from Arthrobacter sp. TE1826. The gene was sequenced and found to encode 289 amino acid residues (M.W. 32,746). A protein homology search revealed that the soxR gene product belongs to the LysR family of regulatory proteins. When soxA was cloned in Escherichia coli, the gene was constitutively expressed. In contrast, sarcosine was required as an inducer for the expression of soxA in the parental Arthrobacter strain. When soxA and soxR were simultaneously cloned in E. coli, sarcosine oxidase production was drastically decreased. This strongly suggests that SoxR acts as a negative regulator for soxA.