Pseudomonas aeruginosa biomass was used to investigate the biosorption properties of heavy metals in wastewater. The biosorption isotherm of Co2+ was best described by the Langmuir model when washed cells were employed, and results obtained utilizing heat-treated P. aeruginosa were also adequately represented by a Langmuir sorption isotherm. In contrast, the sorption isotherm involving unwashed P. aeruginosa showed a different isotherm profile and did not attain equilibrium in the range of metal concentrations investigated; the amount of Co2+ uptake increased with increasing initial metal concentration but never reached adsorption equilibrium, most likely due to bacterial production of extracellular polymeric substances (EPS). The biosorption results utilizing unwashed P. aeruginosa were best described by a Freundlich isotherm. The level of metal adsorption in low pH was significantly small due to competition between the cation and H+ ions for binding sites distributed on cell surfaces, while the increase in pH favored metal sorption because of the elevated quantities of negatively charged surface functional groups. The sorption of Co2+ was strongly influenced by the presence of competing cations in the solution. Trivalent Cr3+ added to the solution was preferentially adsorbed onto the cells relative to Co2+ and Ni2+. The results may be attributed to the higher adsorption affinity of Cr3+ in comparison to either Co2+ or Ni2+. The affinity order (Cr3+ > Co2+ » Ni2+) was maintained over a pH range up to 5.3 in a mixture solution.