ABSTRACT: Mercury concentrations were determined in feather and food samples from adults of 6 seabird species from the Azores archipelago. Mean concentrations ranged from 54 to 432 ng g^-1 dry weight in food and 2.1 to 22.3 µg g^-1 fresh weightin feather samples and were highly significantly and positively correlated. Significant seasonal differences in dietary mercury were also registered. Both feather and food mean concentrations were 4-fold higher in seabirds feeding on mesopelagic prey thanin those feeding predominantly on epipelagic prey. Concentrations in feathers were 150x those in food, which represents the highest methylmercury biomagnification factor reported for consumers in aquatic foodchains. These findings show that the inclusionof mesopelagic prey in seabird diet has an influence on mercury accumulation over and above that from trophic status and emphasises further the value of seabird feathers as monitors of the ecological hazards of mercury in marine ecosystems.