The Southampton Water zooplankton community was last studied extensively in the end of the 1950's, and the impact on the community structure of a substantial oil refinery was never practically evaluated. The present research re-evaluted the zooplankton community structure within Southampton Water both in space and in time, over a two year sampling programme. While species composition of the zooplankton community was fairly uniform throughout the estuary, certain trends were evident with calanoid copepods dominating in numbers during the winter months and other species, such as cirripede and polychate larvae, showing preference at either end of the estuary. The ecotoxicological assessment of hydrocarbon pollution involved a synthetic oil, representative of the refinery discharged effluent, whose aromatic hydrocarbons were composed mainly of alkyl benzenes and toluene compounds. The laboratory experiments demonstrated that calanoids and cirripede larvae were affected by 48 h EC₅₀ oil concentrations ranging from 2.41 to 7.88 mg/l, and also displayed an increase in oxygen consumption patterns at the tested concentration of 433 μg/1, over a 72 h period. Considering the effluent dispersion, time of residence within the water column, and known hydrocarbon concentrations in the estuary, it was concluded that the zooplankton community could potentially suffer sub-lethal damage in certain restricted areas of Southampton Water.