Effects of toxicants on estuarine macrobenthic animals that developed in sand-filled boxes in the laboratory and field during 8 weeks were determined by comparing community structures in control boxes and in boxes treated with a toxicant. Communities that developed were diverse and averaged 1441 individuals, 30 species, and 6 phyla for laboratory tests and 933 individuals, 51 species, and 8 phyla for field tests. Toxicants were introduced via water, air, or sediment and before, during, or after colonization. Tests with laboratory- and field-colonized communities provided corroborating data as well as data unique to each test.