Length polymorphism of the mtDNA control region was analysed in samples from the sea bass Dicentrarchus labrax before and after acclimation to freshwater. Acclimation trials were repeated twice for two samples originating from the same broodstock. DNA amplification of 221 individuals made it possible to detect 40 different D-loop length variants and a mean level of gene diversity (k c ) of 0.850. Out of 52 scored distinct genotypes, 18 were homoplasmic and the remaining ones were heteroplasmic for up to four variants. Shifts in frequencies between starting and acclimated samples in the same direction of the same genotypes in both years were not observed. Patterns of mtDNA control region variation were compared to previous allozymic and RAPD studies carried out on the same samples. While variation found in the allozyme alleles and the RAPD markers in replicate experiments revealed differential survival of the genotypes, stochastic processes, such as genetic drift, explained the observed variation of mtDNA control region. Very high levels of genotype diversity were observed within each sample, ranging from 0.77 to 0.97. This high diversity associated with the maternal inheritance of mtDNA makes this molecular marker especially suitable for studies in aquaculture genetics, when the allozyme studies fail to reveal genetic variation or when the identification of progeny based on the mother's contribution is relevant.Le polymorphisme de longueur de la region de controle de l'ADN mitochondrial (ADNmt) a ete analyse dans des echantillons de bar (D. labrax) avant et apres des experiences d'acclimation a l'eau douce. Les experiences d'acclimation ont ete repetees sur deux echantillons de meme origine. L'amplification de l'ADNmt de 221 individus a permis de detecter 40 variantes de longueur de la bouche D et une diversite genique (K c ) de 0,850 en moyenne. Sur 52 genotypes analyses, 18 ont ete homoplasmiques et le reste heteroplasmiques, avec jusqu'a quatre variants chez le meme individu. La variation de la region de controle de l'ADNmt a ete comparee a celle observee lors d'etudes precedentes sur les memes echantillons pour les allozymes et aux marqueurs RAPD. Alors que la variation liee aux allozymes et aux marqueurs RAPD trouves a indique des differences de survie des genotypes apres acclimatation, des processus stochastiques, comme la derive genetique, peuvent expliquer la variation observee. A l'interieur de chaque echantillon, on a observe de hauts niveaux de diversite genetique avec une variation comprise entre l'indice 0,77 et l'indice 0,97. Cette diversite associee a l'heredite maternelle de l'ADNmt semble rendre ce marqueur moleculaire tres utile pour les etudes genetiques appliquees a la pisciculture.