Genetic traits of the lugworm Arenicola marina were determined for 4 Atlantic populations from France to Norway and compared with a population from the sub-arctic White Sea in Russia. Seven loci were analysed using horizontal starch gel electrophoresis. A low heterozygosity (0.09 to 0.17) and a non-significant heterozygote deficiency were found in all populations. The genetic identity between lugworms of European Atlantic populations was high, whereas similarity of the Atlantic populations with the population from the White Sea was low. The gene flow between the Atlantic and the White Sea populations must be considered negligible, as deduced from the average high and significant gene differentiation F(ST). In particular, differences in allele frequencies of glucose phosphate isomerase (Gpi) and phosphoglucomutase (Pgm) showed that the White Sea population differed significantly from the others. A very strong correlation existed between the frequency of the alleles of isocitrate dehydrogenases 2-A and -B (Idh2-A and Idh2-B) and the average water temperature. It is concluded that temperature had a selective influence on isocitrate dehydrogenase 2, which, in contrast to isocitrate dehydrogenase 1, was identified as a mitochondrial enzyme. These findings support the hypothesis that mitochondria play a key role in temperature adaptation and the adjustment of critical temperatures.