The nominal concentration of TBT which inhibited fertilisation was much higher than the ANZECC and US EPA guidelines but Cu inhibited fertilisation at concentrations close to the guideline levels (Table 1). Changes in the concentrations or bioavailibility of Cu and TBT, such as adsorption to the well walls or chemical speciation, may occur once the toxicants are introduced into the assay system. In these scenarios the nominal concentrations found in this study will be overestimates. These results for Cu are consistent with earlier ®ndings for coral fertilisation. An IC50 of less than 100