The purpose of this proposal is to fundamentally understand the effect of structural properties of SWNTs on their fate and biological behavior in the natural aquatic environment. The study design involves systematic evaluation of aggregation, deposition, organic compound sorption, and uptake/toxicity in fish (Japanese medaka) for semiconductive SWNTs for a range of electronic structures (with consequent variation in diameter and chirality). These studies will be fully novel, as there are no fundamental studies reported in the literature examining the effect of basic structural properties on environmental fate and implications of SWNTs. The research proposed will address the following aims. (1) Fractionation of semiconductive SWNTs by diameter/chirality using density gradient ultracentrifugation; (2) examination of aggregation and deposition kinetics of diameter/chirality-sorted SWNT fractions as a function of organic matter concentration and ionic strength using state-of-the-art dynamic light scattering, quartz crystal microbalance, and conventional column-flow transport experiments; (3) determination of the effect of SWNT diameter/chirality on organic contaminant adsorption using common headspace-partitioning methods; and (4) assessment of the uptake, biodistribution, and toxic effects of diameter/chirality-sorted SWNTs in Japanese medaka fish after waterborne- or dietary exposure using near-IR fluorescence spectroscopy and microscopy.