AbstractFrom December 1970 to December 1972, the monthly species composition, density, species richness, and biomass of marine macroscopic invertebrates were quantitatively studied from a wave‐protected, intertidal sand flat at the mouth of Delaware Bay. Considerable erosion, transportation, and deposition of sand occurred throughout the study period. The fauna was extremely flexible and resilient in its response to a gradient of considerable sediment mobility. This response was similar in scope to one recognized along the coast of California. Comparison of the local biota with other sand flats and beaches from Maine to South Carolina showed that (1). haustoriids were among the dominant species, (2). densities were roughly comparable, and (3). secondary species were similar.Based on the biological index value, three species contained 87.8% of the fauna from the Cape Henlopen tide flat. These species were the polychaete Scoloplos fragilis and the amphipod Neohaustorius biarticulatus, which were characteristic of sand bars, and the polychaete Heteromastus filiformis, which was more characteristic of troughs or depressions. Mean density varied considerably both monthly and annually with almost no cyclic repetition for any of the three species. There was evidence to indicate that specific habitat requirements of a few species can strongly affect the structure which emerges for a total fauna. Morphology, behavior, physiology, and reproductive strategy are aspects of biology which link dominant benthic species of intertidal sand beaches together.