AbstractMacrobenthic animal communities that colonized uncontaminated and fenvalerate‐contaminated sand (0.1, 1 and 10 μg/g dry weight, nominal) in boxes placed for 8 weeks in an estuary were compared to assess effects of fenvalerate on community structure. As much as 27% of initial concentrations of this synthetic pyrethrin persisted in sediment at the end of the test. The average number of species (35.6) in communities in five replicates exposed to 10 μg/g was significantly less than that in the control (47.8) and lower concentrations (45.0 and 46.2). Of the dominant phyla collected (Annelida, Mollusca, Chordata and Arthropoda), abundance of chordates only (primarily lancelets,Branchiostoma caribaeum) was reduced by 10 μg fenvalerate/g. Biological indices applied to the data showed the greatest structural differences for communities exposed to the highest concentration, but these did not differ substantially from those for the control. Effective concentration for exposure via the sediment was five orders of magnitude greater than that for waterborne exposure determined in earlier benthic community stu