The effects of physical mixing processes on phytoplankton production in the marine environment are well established. However, the effects of these processes on growth and condition of zooplankton and larval fish are at present poorly understood. In this study, we utilized phytoplankton group- specific fatty acid content to trace the phytoplankton group and mixing regime contributing to the condition of individual juvenile North Sea cod. In order to establish a relationship between lipid tracer content and algal utilization, post yolk- sac larval North Sea cod were reared in the laboratory on food chains based on monocultures of either the diatom Skeletonema costatum or the dinoflagellate Heterocapsa triquetra (algae dominating in the mixed and stratified regions of the North Sea). In the laboratory, these algae were fed to cultures of adult Acartia tonsa, the copepod eggs were collected, hatched and the N1 nauplii from these different feeding regimes fed to post yolk-sac larval North Sea cod. Post yolk-sac larval cod required 8 d on either a Heterocapsa- or Skeletonema-based food chain before tracer Lipid signals (the ratio of the lipids 16:1 omega 7 to 16:0) in the larvae began to change from their original values to those similar to the algae at the base of their respective food chains. The cod larvae displayed a lipid tracer content similar to that of their algal food source after 13 d on their respective feeding regimes. During a cruise in May 1992 to examine the distribution of larval and juvenile North Sea cod, a subsample of 100 juvenile cod from the stratified, mixed and frontal regimes of the northeastern North Sea were examined for their content of lipid biomarkers and condition (as determined by the ratio of total lipid content to total length). Juvenile cod displaying a lipid tracer content indicating utilization of diatom-basea food webs (found in proximity to regions of frontal mixing) were observed to be in significantly better condition (p less than or equal to 0.05) than those containing a lipid signal indicative of utilization of flagellate-based food webs (found in stratified regions of the North Sea).