The aims of the study were to determine the contents of glycogen, organic carbon and nitrogen in the Baltic clam Macoma balthica tissue and to find out whether there is any difference in fitness and chemical composition between individuals living in sediments with different porewater hydrogen sulphide concentrations. It was found that hydrogen sulphide permanently present in the sediment at concentrations exceeding 513 mu mol dm(-3) at the middle depths of the Gulf of Gdansk (60 in) had no adverse influence on the fitness of M. balthica and did not reduce the organic compound content in its tissue. This is very probably because of the presence of oxygen in the near-bottom water. Interestingly, animals living in such concentrations of hydrogen sulphide had heavier shells and a greater soft tissue weight. Moreover, the contents of organic matter and glycogen in their soft tissues were significantly higher. Glycogen was 30% higher on average, ranging from 11.1 to 30.3% AFDW during the year. At all the stations during the year, organic carbon in M. balthica tissue varied from 50.3 to 55.2% AFDW, and organic nitrogen from 7.7 to 11.7% AFDW. (C) 2007 Elsevier B.V. All rights reserved.