the benthic community structure of the colorado river delta, mexico, was studied by digging cores in six points along a line perpendicular to the coastline. three communities were found. the external community is influenced by human activities, but the community indexes are similar to others reported in the literature for equivalent areas. the middle community has the highest density and biomass in the area, apparently because of bioturbation created by the crab uca princeps. the internal community is poor in density and biomass, possibly because of the instability caused by erosion related to the tide flux.