Microplate fluorometric techniques were used to measure ethoxyresorufin O-deethylase (EROD) activity in hepatic microsomes and primary hepatocyte cultures from individual wild double-crested cormorant (Phalacrocorax auritus) embryos. Embryos were collected in 1993 and 1994 from Humboldt Bay and San Francisco Bay (CA, USA) and a reference site in coastal Oregon (USA). Median microsomal EROD activities in embryos collected from San Francisco Bay (in both 1993 and 1994) and from Humboldt Bay (1994) were four- to eightfold higher than the reference site median (Kruskal–Wallis, p < 0.05). This degree of induction suggests that cormorant embryos in the two California locations were exposed to concentrations of dioxinlike compounds that are at the threshold for toxic effects in this species. Substantial variation in the EROD response in cultured hepatocytes was observed between individuals, populations, and the two bird species tested (cormorants and chickens [Gallus gallus]). Although most of the cormorant individuals displayed a consistent dose–response profile, a few individuals were uninducible, showing no appreciable increase over basal activity with increasing dose of inducer. Composite dose–response curves for two cormorant colonies appeared to be divergent in spite of small sample sizes, indicating that inducibility can also vary at the population level. These observations suggest that considerable variability in pollutant metabolism and sensitivity associated with single enzyme systems may exist within wild populations and species.