Risks posed by antifouling agents (irgarol 1051, diuron and dichlofluanid) to the aquatic environment of the Gulf of Napoli have been estimated. Seawater samples were collected monthly in selected harbours and marinas of this area during the boating season (March to November 2005) and off-season (January 2006). Concentration levels have been measured, and the resulting data used to perform a probabilistic ecological risk assessment independently on each biocide. Diuron exhibited higher concentration levels than irgarol 1051 in all the investigated locations: Dissolved concentrations of diuron ranged from <1 to 1,380 ng l −1 , whereas dissolved concentrations of irgarol 1051 ranged from <0.2 to 173 ng l −1 . Contamination appears to be largely dependent on the type and configuration of sampling sites and on the residence time and the density of boats. High levels of booster biocides were associated with marinas housing several recreational water craft and/or fishing boats, whilst commercial ports usually exhibited low concentrations. A seasonal influence was observed, with peak and lowest values found in early summer months and during winter, respectively. The comparison of the respective toxicity benchmarks, expressed as 10th percentiles, suggests that plant species are more sensitive to irgarol 1051 (297 ng l −1 ) than diuron (4,846 ng l −1 ). Based on these conservative effect thresholds for plants, ecological risk from the single investigated biocides can be judged to be low in the study area.