Populations of the springtail Orchesella cincta that live in metal contaminated soils have developed tolerance to cadmium by increased metal retention in the midgut epithelium and excretion at every moult. Regulation of the MT gene was studied in a tolerant population (Plombières, Belgium) and a laboratory culture. Animals were exposed to a range of concentrations of cadmium in the food (0-1.5 μmol Cd/g food). RNA was extracted after 5-14 days of cadmium exposure and used for Northern blot analysis to quantify MT mRNA. MT expression levels were significantly higher (p < 0.01) in individuals from laboratory-raised strains originating from the soils of the metal contaminated forest Plombières (2.4- to 7.8-fold expression) compared to the reference population (1.5- to 2.4-fold expression). No variable sites were found in the complete MT coding sequence. Southern blot analysis suggests that in both populations the gene is not tandemly repeated. This is the first evidence of evolution of metal tolerance via gene regulation of MT in a natural population. These data indicate a higher fitness of the tolerant population in the polluted environment due to selection of high MT expression phenotypes. © 2003 Elsevier Science Ltd. All rights reserved.