The present work was set as a part of the evaluation of the health status of the Atlantic coastline from the El Jadida region. This study was organized to study the reproduction cycle of the Mytilus galloprovincialis mussel, based on the technique of measuring the oocyte diameter and the measurement of biometrics. Thus, about 30 mussels in the size class (4-5) cm were collected in bi-monthly increments from January to December 2019. Two sampling sites were selected: the Jorf-Lasfar site (J) and the Haouzia site (H). The physic-chemical parameters (temperature and pH) were recorded for the water of the study sites. The results obtained indicate that the mean oocyte diameter follows an evolution managed by the reproductive stages of M. galloprovincialis from both sites. The minimum oocyte diameter values were 20.5µm and 24.25µm found in mid-May, respectively, in J and H. This is the main stage of gamete emission; the secondary emission was noted in August/September and November/December. Variation in the condition index and gonad index supports this study. The reproductive cycle of M. galloprovincialis is therefore bisexual. The technique of monitoring oocyte diameter measurement is useful and very practical for studying the reproductive cycle in mussels.