The influence of different cationic pretreatments of biomass (2000 meg cations/g biomass) on the biosorption of cobalt on Pseudomonas halodenitrificans was investigated. Li+ was the least inhibitory of Co2+ biosorption (0.64 meq/g) and divalent ions were the most inhibitory (0.56 meq/g with Ca2+,0.46 meq/g with Mg2+). Nevertheless, the binding between monovalent ions and biomass was too weak to study the role of ion-exchange in the mechanism of biosorption. A stoichiometric exchange occurred between Ca2+ and 1 mM Co2+ or less.