The biosorption of the heavy metals most frequently, found in polluted environments by Pseudomonas aeruginosa and Bacillus thuringiensis was studied The effects of these metals on bacterial growth, quantity of dry cells, ammonium assimilation pigment production, and protein synthesis were investigated. At lower concentrations than the minimal inhibitory concentration (MIC), the metals partially limited bacterial growth and caused an inhibition proportional to the metal concentration applied. The production of bacterial biomass varied according to the nature and concentration of the metals, and to the bacterial strain studied The biosorption of metals by P. aeruginosa and B. thuringiensis was variable. Mercury and copper appeared to be the elements most adsorbed by bacteria. Citrate noticeably increased the biosorption of chromium by P. aeruginosa (0.07-45.9%) and copper by B. thuringiensis (18.7-33.8%). Metallic cations everted variable effects on protein synthesis. Zinc stimulated protein synthesis in P. aeruginosa, and cadmium inhibited it significantly in B. thuringiensis. Mercury and cobalt, at concentrations below the MIG, always inhibited the synthesis of pigments in P. aeruginosa. The strong interactions of mercury and copper with organic matter suggest that these undesirable elements might be removed from the environment by bacterial trapping and sequestration. A better understanding of the different for-ms of metals actually existing in polluted environments (speciation) would be of gr eat interest.