The influence of coal ash effluent upon the densities of macrobenthic invertebrate and mosquitofish (Gambusia affinis) populations in a swamp drainage system was studied for a period of 50 mo. Density of the aquatic biota was periodically altered by three major perturbations: (1) heavy ash siltation from inefficient basin operation, (2) lowered pH from the fly ash addition, and (3) coal ash associated elemental concentrations. Siltation appeared to be most influential in reducing the invertebrate densities while low pH (mean change from 7.2 to 5.5, extreme of 3.5) was more effective in reducing mosquitofish populations and retarding the recovery of invertebrates. Dipterans (chironomids) and odonates (Plathemis lydia and Libellula spp.) were the invertebrates most tolerant to coal ash stress. Upon the completion of an efficient primary–secondary retaining basin system, most invertebrate groups in the swamp were able to recover to a level of abundance equal to or greater than that which existed 4 yr earlier. Key words: coal ash stress, heavy ash siltation, fly ash acidity, elemental accumulation, benthic macroinvertebrates, Gambusia affinis