NOAA’s Environmental Sensitivity Index (ESI) lists saltmarshes along the Atlantic and Gulf Coasts as the most vulnerable habitat to oil spills. As a result of the Deepwater Horizon offshore drilling rig accident, more that 200 million gallons of crude oil were released into the Gulf of Mexico, and more than 1.8 million gallons of dispersant were applied in the response effort. While previous studies have characterized the toxicity of crude oil and dispersants in aquatic species, there is a need to better understand potential impacts of both crude oil and dispersants on sensitive saltmarsh communities. The purpose of this project was to assess acute and chronic impacts on various biological constituents of a simulated (mesocosm) saltmarsh community. The project was conducted in two phases, with Phase I focused on simulating the impacts of a single incoming crude oil slick. Treatments included crude oil only, dispersant only and a crude oil+dispersant mixture. Phase II simulated multiple re-oiling events with incoming tides and included the same treatments as in Phase I. Results from Phase I showed that oil and dispersant concentrations in water were significantly elevated at 12-24 h in both the oil only and oil+dispersant treatments, but measured concentrations declined rapidly thereafter. Oil concentrations in sediments were slightly elevated at 30 and 60 d in the oil only and oil+dispersant treatments. No treatment-related effects were observed on the survival of fish, molluscs, or crustaceans. Likewise, no significant effects were observed in the benthic community or on the growth of marsh grass. Phase II results showed that waterborne oil concentrations were significantly elevated at 7 d in all oil and oil+dispersant treatments. Oil concentrations were significantly elevated in sediments for the oil only and oil+dispersant treatments. Survival of clams was significantly reduced in all oil and dispersant treatments. Growth of marsh grass was reduced in both the oil only and oil+dispersant treatments. A trend toward decreased benthic community diversity and total densities in the oil only and dispersant only treatments was observed after 7 d. No significant effects on the survival of fish or grass shrimp were observed; however, ethoxyresorufin-O-deethylase (EROD) activity (a biomarker of polycyclic aromatic hydrocarbon [PAH] exposure and uptake) was induced in the fish in both the oil only and oil+dispersant treatments at 7 d. Reductions in dissolved oxygen were observed throughout most of the exposure (~ 20 d) in the oil only and oil+dispersant treatments, and were most extreme in the oil+dispersant treatment. The results of this study highlight the utility of a mesocosm approach to identify impacts associated with a complex environmental exposure scenario, like a crude oil spill with and without a chemical dispersant, in various compartments of estuarine ecosystems.