The copepod Calanus finmarchicus is a marine ecological key species in the Northern Atlantic food web. This species was exposed to an artificially weathered North Sea oil dispersion (oil droplets and water-soluble fractions [WSF]) and a filtered dispersion (containing only WSF) in serial dilution. Female copepods were divided into lipid-rich and lipid-poor for each exposure followed by gene expression analyses of glutathione S-transferase (GST) and cytochrome P-450 330A1 (CYP330A1). Lipid-rich copepods exhibited elevated transcription of GST and reduced transcription of CYP330A1 after exposure to both dispersed oil and WSF. In contrast, lipid-poor copepods exhibited increased transcription of CYP330A1 following exposure to WSF but not the dispersion. Data suggested that small lipid storage promotes increased bioavailability of accumulated oil compounds. Variations in response in CYP330A1 gene expression indicate that oil constituents may exert different modes of toxic action in copepods depending on their reproductive stages. The contribution of oil droplets to the observed effects seemed to be low as GST gene expression was similar after exposure to both dispersed oil and WSF. However, feeding rate in copepods exposed to dispersed oil was reduced, and this may have decreased the uptake of oil constituents via the diet. Although quantitatively higher mortality was observed in copepods exposed to the highest dispersion levels, this may result from smothering of animals by oil droplets. Furthermore, increasing dilution of both the dispersions and the WSF altered their distributions and chemical composition, which may influence the bioavailability of spilled crude oil to pelagic marine organisms.