Measurement of acetylcholinesterase (AChE) is widely used as a biomarker of exposure to neurotoxic compounds in aquatic environments. In the present study, AChE activity was monitored in the clam Tapes philippinarum in the Lagoon of Venice, with the aim of evaluating the possible exposure of animals to neurotoxic compounds. Two sampling strategies were followed. In the first case, temporal and spatial variations of AChE activity were measured in clams collected seasonally (from October 2003 to June 2004) in 3 sites of the Lagoon: Campalto (site 1), near a sewage treatment plant, Marghera (site 2), a highly polluted area, and Poveglia (site 3), a reference site. The condition index (CI) of clams was also calculated. In the second case, AChE activity was measured in clams collected at the end of seasonal samplings in 12 sites located throughout the Lagoon: areas licensed for clam farming (sites 5 to 11), nearby canals characterised by waste waters from agricultural areas (sites 4, 12, 13, 14), and a reference site outside the Lagoon (site 15). Results revealed a significant reduction in AChE activity in clams from Marghera in October, January and April, indicating their probable exposure to neurotoxic compounds originating in the industrial zone of Porto Marghera. A seasonal trend in AChE activity was also observed, enzyme activity being higher in January and lower in June. Clams from Campalto generally had higher CI values than those from Marghera and Poveglia. However, no particular correlation between AChE activity and CI was found. Lastly, marked differences in AChE activity were recorded when comparing enzyme activity of clams from various sites in the Lagoon of Venice with those of clams collected at site 15. Significant reductions in AChE activity were observed in animals collected in both nearby canals and licensed areas, indicating the homogeneous spatial distribution of potentially neurotoxic compounds throughout the Lagoon. It is probable that sediment mobilisation caused by clam harvesting and the passage of shipping plays an important role in resuspension of persistent contaminants having neurotoxic activity.