The breeding cycle of scallops (Pecten maximus L.) from a population harvested in Bay of Saint-Brieuc has been surveyed over a period of three years with a classical gonad index, which is useful for the detection of the maturity and first spawning at the beginning of the season. Then phenomena are more complex with rematuration and late spawning.So we tried to find a new biochemical index and in 1977 the in situ sexual maturation of individuals from this population was studied simultaneously by measuring concentration of aspartate transcarbamylase (ATC), which was used as a potential index of gonad growth.A positive correlation was evident between this activity and the growth rate of the gonad.During the spawning period, the variations of ATC activity are described as a function of the maturity stages of the gonad. [NOT CONTROLLED OCR]