The genes for resistance to any essential metal ion are generally tightly regulated. In Pseudomonas putida strain S4, a multiple metal-resistant strain, mutational analysis gave strong evidence to the presence of the same for the expression of Zn resistance. Zn-sensitive mutants showed a lower MTC of Zn and expressed the Zn resistance genes with a lower efficacy. Non-complementation between these mutants suggests that they are possibly involved in the same function. Altered response to Zn of these mutants assisted in predicting the involvement of a repressor protein regulating the expression of Zn resistance genes. Zn hypersensitive mutant, on the other hand, appears to have an unregulated Zn uptake. This seems to provide the sensor component in the regulation. Zn resistance in strain S4 consists of three steps, viz., uptake, efflux, and binding, which are shared by a Zn homeostasis mechanism as well.