Total mercury and mercury species (methylmercury-MeHg, inorganic mercury--Hg(2+)) were determined in the aquatic ecosystem Záhlinice (Czech Republic). Four tissues (muscle, intestines, liver and kidney) of three bird species--cormorant, great crested grebe and Eurasian buzzard, muscle tissues of common carp, grass carp, northern pike, goldfish, common tench, perch and rudd, aquatic plants (reed mace and common reed), sediments and water were analysed. Relative contents of MeHg (of total Hg) were in the range from 71% to 94% and from 15% up to 62% in the muscle and intestines and in liver, respectively, for all birds. Statistically significant differences were found between contents of MeHg in liver tissues of young and adult cormorant populations (F(4.60)=56.71, P<10(-5)). Relative contents of MeHg in muscle tissues of fishes were in the range from 65.1% to 87.9% of total Hg.