The technology of microbial removal of ionic mercury from wastewater of chloralkali electrolysis amalgam process has been presented in this paper. The development of this biotechnology, starting from laboratory investigations of the bioreduction of ionic mercury to insoluble metallic mercury, through the screening of mercury resistant microorganisms (with expression of mercuric reductase) and kinetic studies of the process, employment of bioreactors with fixed-bed and fluidized-bed of immobilized microorganisms, up to the industrial impelemnetation have described. The results of laboratory experiments, as well as from pilot and industrial plant in Spolchemie in Czech Republic, have been demonstrated in order to depict the advantages of this technology. This robust bioremediation method allows to reduce ionic mercury concentration from 10 mg/l to the level of 50 mg/l which is the solubility level of metallic mercury in water. The technology is applicable not only to wastewater from chloralkali electrolysis plants, but also to other types of mercury polluted water e.g. gold mining wastes, leachates and off-gases wash water.