Macrobenthic animal communities that colonized sand-filled aquaria were exposed to 1,2,4-trichlorobenzene (TCB), a recent replacement for polycholorinated biphenyls in the electrical industry. In one test, communities established by planktonic larvae entrained in continuously supplied unfiltered seawater for 50 days were exposed to waterborne TCB for 6 days; in the second test, the toxicant was added to the sediment before 8 weeks of colonization. Concentrations that affected community structure were usually two orders of magnitude lower for waterborne TCB than for sediment-bound TCB, but the same types of organisms were affected by each route of exposure. The lowest TCB concentrations (measured) that affected average numbers of individuals exposed via the water were 0.04 mg/liter for mollusks, 0.4 mg/liter for arthropods, and 4 mg/liter for annelids. Average number of species was significantly lower than the control at 4 mg/liter. For TCB exposures via the sediment, the lowest concentrations (nominal) that affected average numbers of individuals were 100 micrograms/g for mollusks and echinoderms, and 1000 micrograms/g for arthropods and annelids. Average number of species in experimental aquaria was significantly lower than the control at greater than or equal to 100 micrograms/g. TCB persisted in sediments, but some leached into water throughout the 8-week exposure via sediment.