The toxicity and bioaccumulation of fly ash constituents (fly ash particulates, pH excursions and heavy metals) in three species of stonefly nymphs (Plecoptera), Tallaperla maria, Pteronarcys dorsata and Pteronarcys proteus were investigated in laboratory and field located bioassays. Fly ash particulates were non-toxic to the three species. All three species were highly resistant to pH extremes with acidic 96-hr LC50 values ranging from 2.8 to 3.3 and alkaline 96-hr LC50 values ranging from 12.1 to 10.4. Exposure of P. proteus to low pH (3.0) for 120 hrs resulted in a loss of body sodium. Both acidic and alkaline pH extremes caused ultrastructural changes in the gills of P. dorsata. The toxicity of cadmium to P. proteus increased with length of exposure. The uptake of cadmium and zinc by P. proteus was investigated in laboratory flow-through artificial streams. Both metals were accumulated above control levels in the environmental exposures. Maximum cadmium accumulations occurred between day 7 and 14 at 7 and 21 C. The maximum body burdens of zinc, however, were not obtained until day 28. Temperature had no significant effect on the uptake of either metal. Exposure to 5.0 mg/L cadmium for 14 days caused substantial changes in the chloride cells of P. proteus. Cadmium was concentrated to the greatest extent within the internal tissues of P. proteus. The bioaccumulation of six heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) by P. proteus exposed to a simulated heavy metal effluent was investigated in field located artificial streams receiving natural river water. All the metals except zinc were accumulated above control levels, with body burdens of each of the metals reaching peak concentrations between day 7 and 28 of exposure. / Master of Science