The aims of the present study were to examine some aspects of reproductive biology, and analyze the relationship between length-length and length-weight of grey mullet Mugil cephalus sampled between February 2017 and January 2018 at several localities in South Atlantic Morocco. The individuals' weight and length were measured. The sex ratio of Mugil cephalus was 1, 0.46. The total length (L_t) at first maturity (L_t50) was estimated for both sexes. The spawning period of M.cephalus in South Morocco occurs most of the year in females and is focused in summer in males. The morphometric relationships between the different lengths TL (total length), SL (standard length), and FL (fork length) are linear for females, males and sexes combined. SL = 0.802TL + 13.608* TL = 0.9007 FL - 4.8959*FL = 0.8554SL + 32.911. The length-weight relationships were established for both sexes: WT= 0.00002*TL^2.89 (R² = 0.92), for females WT = 0.0007*TL^2.692 (R² = 0.86), and for males WT = 0.0003*TL^2.882 for males (R² = 0.872). Results showed that the grey mullet had negative allometry. Length grows more than weight in the South Atlantic Coast of Morocco. In addition, the parameters from von Bertalanffy's growth equation were estimated for Mugil cephalus, where L∞ = 804 mm; W∞ = 4979.82g; k =0.14, and t0 = –0.4949.