Aquaria containing clean sand received a continuous supply of flowing seawater from Santa Rosa Sound, Florida, mixed with known quantities of Dowicide G-ST(79% sodium pentachlorophenate) for thirteen weeks. The measured concentrations of pentachlorophenol (PCP) in the aquaria were 1.8, 15.8, and 161 micrograms/l. At the end of the experiment, macrofauna established in control and experimental aquaria was examined. Mollusks, arthropods and annelids were numerically dominant among the macrofauna. Although exposure to 1.8 micrograms PCP/l had no effect, the higher concentrations of PCP caused marked reduction in the numbers of individuals and species. Mollusks were the most sensitive taxonomic group to PCP. These results and previous studies on the effects of a nine-week exposure to PCP on the establishment of macrobenthic communities indicate that discharge of PCP into natural waters could alter the normal colonization by benthic animals and could impact various ecological relationships among localized populations.