Manufactured nanoparticles (NPs) can associate with toxicants in the aqueous phase and these associations can influence the environmental fate, transport, and bioavailability of these toxicants in organisms. Dissolved metals (e.g., Hg2+) can be toxic in aquatic organisms, and, if metals associate with NPs in the aqueous phase, changes in bioavailability and toxicology may result. Here we demonstrated that Hg2+ (25 mu g/L) can associate with aqueous (C-60)n (termed nC(60)) and increase aggregate size and settlement of nC(60) aggregates out of the water column over 24 h. The concentration of C-60 was directly related to concentration of Hg for nC(60) aggregates that settled to the bottom of the container. Bioavallability of Hg2+ in larval zebrafish Danio rerio, evaluated by assessment of metallothionein gene (mt2) expression, was reduced in the water column when nC(60)was present. However, zebrafish residing at the container bottom and exposed to nC(60) aggregates with associated Hg2+ had elevated expression of mt2 when compared to fish exposed to 25 mu g/L Hg2+. preparations without nC(60), which indicated nC(60) led to a localized increase in Hg2+ bioavailability. Results indicate that aqueous nC(60) can sorb Hg2+, transport Hg2+ to substrate surface; and increase concentrations of bioavailable Hg2+ in organisms located where settled nC(60) aggregates accumulate.