Pure microcosm-cultured populations of benthic copepods were established from pristine or pesticide-impacted Spartina marsh creeks and used as efficient bioassay groups to assess lethal and sublethal effects of sediment-bound pesticide residues. Despite a broad data base showing extreme sensitivity to water-solubilized fenvalerate by many marine invertebrates and fishes, a 7-day exposure to sediment-bound residues as high as 100 ppb caused no significant mortality for any life stages (i.e., nauplii, copepodites or adults of the benthic harpacticoid copepods Microarthridion littorale or Paronychocamptus wilsoni, and no mortality for adults of Enhydrosoma propinquum. However, sediment-bound residues as low as 25 ppb significantly depressed egg production (50-100% reduction) and mean clutch sizes (40-100% reduction) of fertile M. littorale and P. wilsoni. If sedimenting fenvalerate depresses copepod reproduction in the field, then lowered recruitment of new individuals will lead inevitably to a decline in population growth. (Copyright (c) Elsevier Science Publishers Ltd., England.)