The impact of two formulations of chromated copper arsenate (CCA) wood preservatives (Celcure AO and Tanalith C33 10) on a range of marine organisms was assessedin both laboratory and field investigations. As part of an EU project, a long-term field investigation was carried out exposing test panels of Scots pine treated to 12,24 and 48 kgm -3 CCA and untreated controls at seven coastal sites (Portsmouth, UK; Kristineberg, Sweden; La Tremblade (2 sites), France; Ria Formosa, Portugal; Sagres, Portugal; Athens, Greece). Inspections were made at 6,12 and 18 months after submergence and the fouling communities on CCA-treated and untreated panels were assessed both quantitatively and qualitatively. Over the course of the field trial patterns of fouling that were common at several of the sites were recorded. In the majority of cases there were no significant differences in species abundance, species diversity, community organic biomass and total dry weight of fouling organisms on CCA-treated versus untreated panels. However, a number of species were recorded in significantly higher numbers on CCA-treated panels compared to untreated panels. The extent and type of microbial degradation of these panels was also assessed. Sections from the panels exposed for periods of 6,12 and 18 months were examined by light microscopy and the depth of penetration by soft rot fungi and tunnelling and erosion bacteria was measured. Attack was found to be heavy in untreated wood, but absent or very superficial in CCA-treated wood. The fungal flora was also recorded after 6,12 and 18 months exposure following incubation of panels. Fungal diversity on untreated wood was high, whereas on CCA-treated wood fungi were either absent, or a low diversity present. The surfaces of untreated panels were notably softer as a result of microbial and marine borer attack. Scanning electron microscope studies revealed that the rate of biofilm formation on the surfaces of CCA-treated panels (12,24 and 48 kgm -3 CCA) was marginally faster than on the surface of untreated panels over a4 week exposure period in the sea at the Portsmouth exposure site. After 4 weeks of exposure no differences in biofilm composition were detectable between different panel treatments. Algal fouling was monitored on CCA-treated and untreated Scots pine panels after submergence at the exposure site at Portsmouth, UK for a period of 4 weeks. In the majority of cases there were no significant differences in the abundance of each algal species on CCA-treated or untreated panels. Two species of algae were recorded in significantly lower abundance on CCA-treated panels compared to untreated panels, suggesting that these species were sensitive to leached preservative from the surfaces of the treated panels. The impact of CCA-preservatives on invertebrate fouling was also monitored over the initial 4 weeks of submergence. Scots pine panels treated to 2,4,6,12,24 and 48 kgm -3 (Celcure AO and Tanalith C3310), and untreated controls were exposed at a brackish water millpond, Emsworth, UK. Settlement densities of 3 species of calcareous invertebrates were measured. Densities of the serpulid Ficopomatus enigmaticus significantly increased with increasing CCA loading; numbers of the two species of barnacle (Elminius modestus and Balanus crenatus) were also higher on CCA-treated panels than on untreated panels but these differences were often not significant. A laboratory leaching trial combining a static and a flowing seawater system was carried out to measure the leaching rate of copper, chromium and arsenic from the surface of Scots pine panels treated to 3,6,12,24 and 48 kgm-3 CCA. Losses of chromium and arsenic over the 10 week trial were very low, whereas losses of copper were much higher. Copper losses from freshly treated panels were highest, but rapidly fell to much lower levels after exposure in flowing seawater. Laboratory bioassays were carried out to test the toxicity of leachates from CCA-treated wood, or their toxic components. Tests on the thraustochytrid, Schizochytrium aggregatum, examined the effects of copper ions on mortality of sporangia and motility and viability of zoospores. A very high tolerance to copper ions was recorded in both instances. Experiments exposing spores/zygotes of two species of marine algae found that concentrated leachates from CCA-treated wood inhibited germination of Fucus serratus zygotes but only reduced the percentage germination of Undaria pinnatifida spores. All of the bioassays used produced rapid results. Data suggests that CCA-treated wood is not toxic to non-target marine fouling organisms. Preferences to settle on CCA-treated wood over untreated wood by certain marine fouling organisms may be due to differences in the surface properties of the wood (surface energy, biofilm formation) influencing choice of settlement sites by fouling propagules. Differences manifested at this initial stage of the fouling process may be reflected in the mature fouling community.