Wastewater chlorination may reduce the number of pathogens in wastewater effluents and may beneficially impact the microbiological quality of waters receiving these chlorinated discharges. However, there is concern that current wastewater chlorination practices may not be effective in achieving the above goals and may, in fact, produce toxic chlorination by-products that are detrimental to the water quality of the receiving water. Halogenated organic (halo-organic) compounds are one class of wastewater chlorination by-products that are of environmental and public health concern. The formation of halo-organics during drinking water chlorination has been heavily researched, but less research has been devoted to the formation of these compounds during wastewater chlorination. The research described in this report involves an investigation of halo-organic formation during wastewater chlorination in field and laboratory studies. The field studies focused on the formation of halo-organics at several locations after chlorination at three wastewater treatment plants. Total organic halogen (TOX), THMs, and the residual chlorine were monitored at each sample location with samples of unchlorinated wastewater analyzed for organic content and ammonia concentration. Even though the chlorine dose, organic content, and ammonia concentration were different at these facilities, the results were similar. The results showed that TOX was present in the unchlorinated wastewater and that additional TOX was formed immediately after chlorine addition. TOX formation did not increase with increasing contact time. THM formation was not significant at any of the treatment plants. The amount of TOX formed as a result of wastewater chlorination was not significantly greater than the amount of TOX present in the unchlorinated wastewater. The laboratory experiments were conducted in order to investigate the influence of chlorine dose, contact time, initial mixing intensity, and ammonia concentration. At chlorine doses similar to those commonly encountered during wastewater chlorination, TOX formed immediately after chlorine addition, but no additional TOX formation was detected with increasing contact time. TOX and THM formation were sensitive to the ammonia concentration when chlorine-ammonia ratios were greater than 1 mol/mol (above the "hump" of the breakpoint curve), but not when ammonia was in excess (below the "hump").