Macrobenthic animal communities, developed in sand-filled aquaria in the laboratory and in the field, were exposed to various concentrations of the insecticide chlorpyrifos, and effects on community structure were assessed. Laboratory communities were continuously exposed to the toxicant for 8 wk during colonization by planktonic larvae in unfiltered Santa Rosa Sound, Fla., seawater. Field communities were developed for 8 wk in aquaria placed in Santa Rosa Sound, then removed to the laboratory for exposure to chlorpyrifos for one week. Abundance of arthropods was significantly diminished (alpha = 0.05) by measured concentration of chlorpyrifos greater than or equal to 0.1 microgram/l in water in laboratory communities and by 5.9 microgram/l in water in field communities. Numbers of annelids and chordates in contaminated aquaria were not reduced by the highest concentrations of chlorpyrifos tested, 8.5 microgram/l in laboratory-colonized aquaria and 5.9 microgram/l in field-colonized aquaria. One species of annelid, Cistenides gouldii, was more abundant in field aquaria receiving 1.0 microgram/l or 5.9 microgram/l than in the control and lowest concentration. Molluscan larvae colonizing laboratory aquaria were sensitive to greater than or equal to 0.1 microgram/l; however, later developmental stages characterizing field aquaria were not sensitive to less than or equal to 5.9 microgram/l. Although only 20 of 78 animal species appeared in both laboratory and field communities, sensitivity of animals in these tests and in single species tests could be compared.