Hydrocarbon-degrading bacteria are important for controlling the fate of natural and anthropogenic hydrocarbons in the marine environment. In the wake of the Deepwater Horizon spill in the Gulf of Mexico, microbial communities will be important for the natural attenuation of the effects of the spill. The chemical dispersant COREXIT® EC9500A was widely deployed during the response to the Deepwater Horizon incident. Although toxicity tests confirm that COREXIT® EC9500A does not pose a significant threat to invertebrate and adult fish populations, there is limited information on its effect on microbial communities. We determined the composition of the microbial community in oil that had been freshly deposited on a beach in Louisiana, USA, as a result of the Deepwater Horizon spill. The metabolic activity and viability in cultures obtained from oil samples were determined in the absence and presence of COREXIT® EC9500A at concentrations ranging from 0.001 to 100 mg ml–1. In length heterogeneity PCR (LH-PCR) fingerprints of oil samples, the most abundant isolates were those of Vibrio, followed by hydrocarbon-degrading isolates affiliated with Acinetobacter and Marinobacter. We observed significant reductions in production and viability of Acinetobacter and Marinobacter in the presence of the dispersant compared to controls. Of the organisms examined, Marinobacter appears to be the most sensitive to the dispersant, with nearly 100% reduction in viability and production as a result of exposure to concentrations of the dispersant likely to be encountered during the response to the spill (1 to 10 mg ml–1). Significantly, at the same concentration of dispersant, the non-hydrocarbon-degrading Vibrio isolates proliferated. These data suggest that hydrocarbon-degrading bacteria are inhibited by chemical dispersants, and that the use of dispersants has the potential to diminish the capacity of the environment to bioremediate spills.