Fly ash is one of the most abundant waste materials; its major components make it a potential agent for the adsorption of heavy metal contaminants in water and wastewaters. The objective of this study was to measure the changes in toxicity of and heavy metals in a municipal (Salisbury, MD) wastewater treatment plant (SWTP) effluent on treatment with fly ash. The effluent from SWTP after treatment with fly ash for 4 h showed a significant reduction in toxicity, Cu and Pb, and PO43− and NO3− contents. Toxicity of the effluent was measured using the marine luminescent microorganisms—Vibrio fischeri (‘Microtox’ test). Heavy metals (Cu, Pb and Zn) were measured using Atomic Absorption Spectrophotometry. Fly ash removed Cu and Pb from the effluent (through adsorption), and the removal of these toxic heavy metals resulted in the reduction of toxicity.