Shell structure of P. imbricata exposed to 270 g L−1 lead was visibly altered, with a complete lack of imbricate processes (frills) around the shell margin. Little information exists on the effects of pollutants on shell growth (Shuster & Pringle 1969, Frazier 1976,Strömgren 1982, Almeida et al. 1998, Beeby et al. 2002), and this is the first study to investigate the effects of pollutants on biomineralization in an economically important pearl oyster. Lead exposure has previously been shown to reduce shell thickness in the common garden snail Helix aspersa (Beeby et al. 2002), and alter amino acid composition of the shell of Crassostrea gigas (Almeida et al. 1998). In our study, shell growth was completely impeded by exposure to 270 g L−1 of lead. Shuster and Pringle (1969) reported complete inhibition of shell growth by Crassostrea virginica exposed to similar concentrations of cadmium, whereas Strömgren (1982) observed cessation of shell growth in Mytilus edulis when exposed to copper and mercury. These results could be because of reduced activity of the enzyme carbonic anhydrase,an enzyme essential for shell and pearl production (Wilbur & Jodrey 1955, Freeman 1960, Miyamoto et al. 1996). Lead has previously been shown to inhibit levels of carbonic anhydrase in both anemones and corals (Gilbert & Guzman 2001), cadmium inhibits carbonic anhydrase in estuarine crabs (Vitale et al. 1999,Skaggs & Henry 2002) and eels (Lionetto et al. 1998), whereas silver, copper and zinc all reduce levels of carbonic anhydrase in crabs (Skaggs & Henry 2002).