With the rapid development of nanotechnology, nanoscale elemental selenium (Se) particles (SeNPs) have been applied for various uses, especially in biomedicine, electronics, catalysis, and food supplement production. Thus, there is increasing concern for SeNPs as emerging contaminants released into the environment. Due to their unique physical and chemical properties, Se nanoparticles (<100 nm in diameter) may pose potential adverse impacts on the environment. However, little is known about the chemical behaviors of SeNP contaminants in the environment. In particular, the biological processes that control the transport and fate of SeNPs in the soil-plant system have not been well elucidated. Some major research questions need to be answered experimentally. Will SeNPs in soil be bioavailable to soil microbes and plants? Will SeNPs be biologically transformed into other Se compounds after their release into the soil-plant system? We speculated that SeNPs could be significantly biotransformed in the soil-plant system by microorganisms such as fungi or soil bacteria that associate with certain plant species. This presentation will report some of research findings obtained from a series of case studies conducted in recent years.