Horseshoe crabs, Limulus polyphemus (L.), are abundant in the lower Delaware Bay during their spring spawning activity. The impact of predation on the benthic community of intertidal flats in the vicinity of breeding beaches was investigated using exclosures and enclosures. In 1978 and 1979, protected sediments contained significantly more individuals, biomass and species per core than unprotected sediments. In 1978, the bivalves Gemma gemma, Mulinia lateralis, and Mya arenaria were significantly more numerous within cages. At that time, unprotected sediments contained very few Mya or Mulinia over 4 mm, suggesting that predators “cropped off” these species as they attained a larger size. Changes in the benthic community were evident when the population of Limulus was at or near maximum. Alternative explanations for the changes seen in the exclusion experiment, including sedimentary modifications and the effects of alternative predators (crabs, fish, and shorebirds), were rejected. Sedimentary modifications were not observed, while the effects of predators besides Limulus were minor.