Counting annual rings on shells, under favourable climatological conditions, usually permits determination of the age of many bivalves. But the application of this method becomes impossible when meteorological conditions are highly variable, as in the Bay of Fos. Annual winter rings are, in fact, often merged with other rings induced by important physiological processes like gametogenesis. When, therefore, annual rings and pseudoannual rings occur, it is impossible to distinguish between them. SEM offers a good solution to this problem, through the establishment of a scheme of interpretation. This method would also be suitable to identify the two main types of rings that could be confused in Ruditapes decussatus L. of the Bay of Fos, using optic microscopy on large samples. The results lead to reliable estimates of the parameters of von Bertalanffy's growth model widely used by many authors for this species.