Macrobenthic animal communities developed in laboratory and in field aquaria during 8 weeks were exposed to various concentrations of the pyrethroid insecticide, fenvalerate. Laboratory communities, developed from planktonic larvae in unfiltered seawater, were continuously exposed to the toxicant during colonization. Field communities, developed in aquaria placed in Santa Rosa Sound, FL, were exposed in the laboratory to fenvalerate after colonization, for 1 week. Abundance of amphipods in both laboratory and field communities was significantly affected by 0.1 microgram fenvalerate/l water (nominal concentration). The lancelate, Branchiostoma caribaeum, also was very sensitive to 0.1 micrograms/l in field communities; it did not occur in laboratory communities. Numbers of mollusks and annelids in contaminated aquaria were not significantly reduced from control numbers by the highest concentrations of fenvalerate tested, 1.1 micrograms/l (measured) in field communities. Of an overall total of 79 species of animals, only 15 appeared in both laboratory and field studies.