Light, in the presence of a photodynamic pigment, induces expression of a specific gene in Pimelobacter (Arthrobacter) sp. The gene encodes a M_r 21,000 polypeptide that accumulates in large amounts on the cell surface. Expression of this gene is also induced when the concentration of free Ca²⁺ ions is lowered by addition of chelating agents. Transfer of cells to the dark or repletion with excess free Ca²⁺ causes synthesis of the polypeptide to return to basal levels. The time course of induction and deinduction by these agents is the same. These results suggest that light and Ca²⁺ interact with a common element in the mechanism that regulates expression of this gene.