Molluscs (Mollusca) belong to one of the most threatened taxonomic groups in the world and mussels (Bivalvia) have been observed to drastically decrease. The reduced growth of the mussel population may depend on factors such as decrease of the host fishes, acidification, invasive species, pollution and siltation of the seabed. All species of large mussels (Unionoida) are sensitive to eutrophication and physical changes in their habitat and can thus be used as indicators of territories with high conservation value or as an indicator of the chemical quality of the water. For a mussel population to be viable it requires a mean the proportion of juveniles (rejuvenation) in the population between 10 to 20 percentages. In Västra Götaland country only half of the previously 33 studied populations show satisfactory rejuvenation. When a mussel is considered to be a juvenile or adult is contentious. Today there are few studies on age at maturity of unionid mussels. Most often estimated values or knowledge of other species age at maturity is used. This work aims to investigate the growth of Anodonta anatina. Growth was measured as the differences in the distance between growth rings on each of the mussel individuals shell. With the annual growth rings an exponential growth rate was calculated that indicated the year in which most of the individuals slowed their growth. The breaking point was found using Von Bertalanffys growth model which indicates growths over time. Knowing the age at which most mussels slows their growth can give us a conclusion of possible age when sexual maturity occurs. The conclusion can be made upon the knowing that an individual starts to allocate the amount of energy available to more features than growth, where reproduction is the most likely. It is important to know the age at which spices reaches sexual maturity due to conservation purposes. A clam is considered very sensitive in its juvenile years. The result in this study shows that growth is slowing at the age of 4 years for one of the studied lakes. In the other two studied, the growth slowed before the mussel attained the age of 3 years. The age at which growth slows differ between lakes and may due to that natural conditions in lakes are various favorable.