Lethal and sublethal effects of 4-nonylphenol (NP) were investigated in the clam Tapes philippinarum from the Lagoon of Venice. In a 96-h lethality test, bivalves were exposed to the following NP concentrations: 0, 0+acetone, 0.19, 0.38, 0.75, 1.5, and 3.0mg NP/L. The 96-h LC(50) value was 1.12mg NP/L. No mortality was observed at 0.19mg NP/L, whereas at 1.5 and 3.0mg NP/L both siphons and foot of clams were often cut-off, the animals being unable to withdraw them before shell closure, suggesting the possible narcotic effect of NP. Lower concentrations (0, 0+acetone, 0.025, 0.05, 0.1, and 0.2mg NP/L) were then used to evaluate sublethal NP effects on clearance rate (CR), respiration rate (RR), scope for growth (SFG), and survival in air. Following a 7-day exposure, decreased RRs were found at all NP concentrations tested. Significant decreases in the CR and SFG were only observed at the two highest NP concentrations (0.1 and 0.2mg/L), with SFG reductions reaching 54% and 71%, respectively. Last, the highest concentrations tested significantly decreased the resistance of clams to exposure to air and enhanced the mortality rate. As the lowest effective NP concentrations are similar to environmentally realistic levels, a condition of potential risk for the well-being of clam populations in estuarine areas is highlighted.