In theory, the higher the covalent index value of a metal ion, the greater its potential to form covalent bonds with biological ligands. Metabolism-independent equilibrium metal ion adsorption to freeze dried Rhizopus arrhizus biomass was found to increase in the order Sr2+ < Mn2+ < Zn2+ < Cd2+ < Cu2+ < Pb2+ and positively correlated with covalent index. Adsorption was rapid and 95% complete within five minutes of metal-microbe contact, and equilibrium was independent of solution biomass concentrations. The potential of a metal to displace other preloaded cations from the biomass ligands, and the extent to which a preloaded ion inhibited the adsorption of another both increased with increasing covalent index for Mn2+, Zn2+, Cd2+, Cu2+ and Pb2+. An almost complete inversion of this order was observed in the case where Sr2+ was the primary binding test ion. According to the hard and soft principle of metal ions, Mn2+, Zn2+, Cd2+, Cu2+ and Pb2+ are classified as soft-borderline, Sr2+ is classified as hard, and theoretically the polar nature of these cations increase in the same order as covalent index. As a consequence of metal ion adsorption, Ca2+ and Mg2+ displacement from the biomass ligands was observed for both hard and soft borderline ions, whereas displacement of H+ was observed for soft borderline ions only. Overall, the soft borderline ions exhibited a significant degree of both covalent and ionic binding, and the hard metal Sr2+ was found to exhibit ionic binding only. Linear reciprocal Langmuir and Scatchard transformation plots reflected the predominantly ion exchange mechanism of Sr2+ and Cd2+ adsorption, and a curved Scatchard transformation plot reflected the more covalent nature of Cu2+ adsorption. The transformation and accumulation of the oxyanion selenite (1000 |imol I'1) by a growing Pénicillium species was investigated over a 2 week period. Selenium in the aqueous phase decreased by ca. 49.8%, and selenium accumulated by the fungal biomass totalled ca. 36.6%. Transformation into volatile selenium compounds amounted to an average value of ca. 8.8%, and the process was determined to be both growth and nongrowth associated. Activated charcoal traps were successfully used to retain the volatile selenium compounds which were determined to be organic in nature. The reduction of selenite to amorphous elemental selenium was observed only during the decline phases of growth. Selenite transformation, particularly reduction to amorphous elemental selenium, was enhanced by the addition of amino acids and vitamins to the aqueous medium, and with such amendments selenite reduction was observed both during the rapid and stationary phases of growth.