The effects of fenthion on macrobenthic animals that colonized sand-filled boxes at two salt marsh sites were determined after two field sprayings within 5 days and in the laboratory after intermittent exposures that simulated field conditions. ULV ground applications for adult mosquito control resulted in water concentrations less than 0.68 micrograms fenthion/l at one site and less than 0.38 micrograms/l the other site. The average numbers of individuals and species of animals that colonized sand-filled boxes in the laboratory (during 9 wk by planktonic larvae from continuously supplied unfiltered seawater) were not significantly affected by two treatments at 1.1 micrograms fenthion/l or by four treatments at 1.3 micrograms/l. However, the average number of species in communities exposed to two treatments at 11.8 micrograms/l was significantly less than that in control communities and those exposed to lower concentrations. Mean abundances of mollusks, arthropods, and chordates were substantially but not statistically significantly, lower in concentration.