Experimental investigations of <biotransformation> of mercury Hg(O) in <aqueous solution> by <immobilized bacteria> were carried out in two types of lab <scale bioreactors>: in fixed and fluized bed. An isolated and identified natural strain of Aeromoans hydrophila was immobilized on the surfaces of porous carriers and employed in mercury removal from liquid phase in the fixed bed reactor. For mercury biotransformation in the fluidized bed a geneticaly modified strain of Pseudomonas putida KT2442:: mer 73 entraped in PVA gel beads was used. The investigations reveal that mercury biotransformation is a fast process controlled by diffusion. The most promising way of mercury removal from waste-water is employment of fluidized bed of immobilized bacteria, because of continuous mercury volatilization and possible recovery of this metal.