(1) Salt-marsh microcosms containing the halophyte Spartina alterniflora were maintained in a greenhouse for 2 years. Microbial and plant activity were monitored monthly in microcosms given daily dosages of a hydrocarbon mixture containing hexane, heptane, octane, nonane, benzene, toluene, ethylbenzene and xylene that comprises about 15% of crude oil. This mixture was added at 3 33 (1 x treatment) and 33 3 g C m-2 dayI (10 x treatment) for 10 months. (2) Aboveand below-ground plant biomass and total respiration rates were significantly increased by the 1 x hydrocarbon additions, and the differences persisted for at least 7 months after the hydrocarbon additions ended. Growth, total sediment organic matter and macro-organic matter were significantly reduced in the 10 x treatment. (3) Soil microbial activities were monitored using non-invasive gas-exchange techniques. CO2 production, methanogenesis, N2 fixation and denitrification were stimulated by the 1 x hydrocarbon treatment but inhibited by the 10 x treatment. (4) Unlike the linear dose-response relationship assumed in many toxicological studies, both plant and microbial activities showed a non-linear response to hydrocarbon concentration in this chronic exposure study, with stimulation of activity at the 1 x level and inhibition at 10 x . The stimulation of microbial activity by the 1 x treatment suggests that the microbes were carbon-limited, while the enhanced plant growth seen at 1 x may have resulted from increased N2 fixation and/or nitrogen mineralization.