The arsR gene encodes the regulatory protein of the plasmid-encoded arsenical resistance operon. A series of in-frame fusions was constructed between the C-terminally truncated arsR gene and the coding region for the mature form of β-lactamase (blaM). Fusions containing most of the arsR gene were still inducible by arsenicals. Fusions containing less than 102 residues of the 117-residue ArsR protein were constitutive. When a wild-type arsR gene was placed in trans, the constitutive constructs were again inducible. The results demonstrate that the ArsR protein is a trans-acting regulatory protein which controls its own expression.