Fifty-two bottom sediment box core samples were taken in fall 1978 and summer 1979 to investigate the relationship between benthic biota and the sedimentary environment of the upper and central Chesapeake Bay. Examination of the vertical distribution of the benthos and radiographic analysis of the sediments determined the degree of physical and biological activities and interactions. The benthic macrofauna were characterized by opportunistic as well as equilibristic species that inhabit a wide range of salinity and substrate types. The number of species increased down the estuary. Greatest species populations were found in the mixed-sediment type environments. Most biogenic structures are produced by polychaetous annelids. Species population patterns and previous Pb210 data in Chesapeake Bay suggest that biological mixing occurs down to 30 cm, being greatest at the sediment surface.