The objective of this study was to evaluate the impact of extreme acidic and alkaline pH excursions of fish populations in streams receiving fly ash settling basin effluent. Fish communities were sampled above and below the effluent outfall in a mountain stream, Adair Run, adjacent to the coal-fired Glen Lyn Power Plant in southwestern Virginia. This effluent, which exhibited a pH increase of up to pH 9.3 as the basin nearly filled, contributed to a downstream decline in species diversity and an increased dominance of the stoneroller (Campostoma anomalum), which displaced more sensitive species. Laboratory avoidance studies, utilizing a steep-gradient trough apparatus, were used to investigate the potential role of avoidance behavior that may alter fish distributions in acidic and alkaline waters. Stonerollers, rainbow trout (Salmo gairdneri), golden shiners (Notemigonus crysoleucas) and spotfin shiners (Notropis spilopterus) exhibited first significant avoidance responses to continuously decreasing pH conditions at pH 6.0-7.0, well above the 96-hr LC50 values of 4.05 and 4.24 for trout and golden shiners, respectively. When the pH was gradually increased, avoidance thresholds of pH 9.5- 11.0 were observed, beyond the 96-hr LC50 values of 9.13 and 8.86 for trout and golden shiners. Single acute alkaline exposures initially yielded more rapid avoidance responses, with acclimation to alkaline conditions frequently occurring within 30 minutes. Gill histological alterations, as observed by transmission electron microscopy, were detected following exposure to extreme pH excursions, with more rapid changes occurring with alkaline exposures. Gill tissue showed marked recovery within 8-14 days. / Master of Science