Pentachlorophenol affected the composition of communities of estuarine organisms developed in sand from planktonic larvae in estuarine water that flowed through ten control aquaria and ten aquaria per exposure concentration averaging 7, 76, or 622 μg/liter. Annelids, arthropods, and mollusks were the numerically dominant phyla when animals were collected in a 1‐mm‐mesh sieve after 9 wk of exposure. Mollusks were markedly fewer at 7 μg/liter; annelids and arthropods at 76 μg/liter. Almost no animals occurred at 622 μg/liter. The total numbers of individuals and species were significantly less (α = 0.01) in aquaria exposed to 76 μg/liter than in those unexposed or exposed to 7 μg/liter.