Metal-tolerant cyanobacteria have been isolated from metal-polluted aquatic environments and also selected in culture, but no genes which confer metal tolerance have been described. To investigate the possibility that amplification of a prokaryotic metallothionein gene (smtA), or rearrangement of the smt locus, could be involved in the development of Cd tolerance in Synechococcus PCC 6301, Cd-tolerant lines were selected by stepwise adaptation of a Synechococcus culture. An increase in smtA gene copy number and the appearance of unique additional smtA restriction fragments (both larger and smaller) were detected in these tolerant lines (tolerant to 0.8 $\mu $M Cd, 1.3 $\mu $M Cd and 1.7 $\mu $M Cd). Stepwise adaptation was repeated by using a culture of Synechococcus PCC 6301 inoculated from a single plated colony to obtain four new lines (tolerant to 1.4 $\mu $M Cd, 1.8 $\mu $M Cd, 2.6 $\mu $M Cd and 3.2 $\mu $M Cd). Amplification of the smtA gene and development of unique smtA restriction fragments (larger and smaller) were once again detected in these tolerant lines. Amplification and rearrangement of the smt locus were only detected in the seven Cd-tolerant lines, with no evidence of amplification or rearrangement in the non-tolerant lines from which they were derived. As a control, another gene, psaE, was also monitored in these cell lines. There was no evidence of amplification or rearrangement of psaE in the non-tolerant or any of the Cd-tolerant lines.