Field experiments in a Maine estuary were designed to follow community development in newly exposed sediment to determine if resident infauna affect the settlement and survivorship of colonizing infauna and thereby effect changes in community structure. Sets of buckets consisting of control buckets (buckets with previously dried sediment) and two experimental treatments (Nereis virens or Glycera dibranchiata added to dried sediment) were exposed on three dates between June and October 1979 and sampled after 8, 16 and/or 24 weeks. At each sampling, new buckets were established. The index of proportionate similarity comparing the infaunal community of exposure periods with the same starting date was usually greater than the index comparing periods initiated on different dates indicating that assemblages of initial colonizers persisted for the length of the experiment despite the availability of potential colonizers. Large numbers of infauna colonized defaunated sediment, while over the same period of time infaunal density usually declined in sediment which originally contained infauna. Effects of disturbance, drying the sediment, on colonization were controlled by comparing densities in sediment exposed on different dates but after the sediment had been exposed 8 weeks, There was no significant difference in net changes in density between these conditioned sediments despite large differences in the density of their residents. Sediment with low initial density, however, always had a greater net change in density than sediment with high initial density, suggesting that residents had some effect on net changes in density. Highest densities of most infauna were recorded in the Glycera addition treatment and lowest in the Nereis addition treatment. Nereis abundance was reduced in the presence of Glycera, which may account for high densities in the Glycera addition treatment. It is important to know the species composition of resident assemblages before it is possible to make accurate predictions concerning effects of residents on colonization. The apparent response of colonists to disturbance, in this study drying the sediment, needs to be controlled in experiments designed to determine effects of residents on colonization.