Sediments frequently cause damage to biota due to the accumulation of toxic compounds and the bioavailability of sediment- associated contaminants. Damage can be assessed using biomarkers, such as the degree of genotoxic impact following in vivo exposure to contaminants. Genotoxic damage, expressed as single- strand DNA breaks, was measured in cells isolated from haemolymph/ blood, gill and digestive gland/ liver from the clam Tapes semidecussatus and turbot Scophthalmus maximus, using the single cell gel electrophoresis ( Comet Assay). Both animals were exposed for three weeks to sediment samples collected from a polluted site and a ` clean' reference site. The level of DNA damage was assessed using an image analysis package and expressed as % tail DNA. Throughout the study, significant differences in DNA damage were recorded for each tissue type, in both species, between animals exposed to the two sediment samples. However, turbot appeared to be a more sensitive indicator species, because, due to lower background levels, they were able to detect a significant difference between reference site and background values. This suggests that turbot, rather than clams, are more suitable as a sentinel species for the assessment of genotoxic impact of low- level contamination in aquatic sediments and highlights the need for a two- or multi- species approach.