Recently, an increasing attention has been paid to the productivity and propagation of grey mullet “Mugil cephalus". In view of the fact that the steroidogenic hormones play a key role in fish maturation and production of fertile ova, the present work was planned to determine and investigate the steroidogenic cells in the testes of grey mullet, and thence to determine their levels of activities during its maturing cycle, aiming at making use of the obtained data in fish breeding and culture. Mature males of M. cephalus were obtained from the natural habitat (Al Bardawil Lagoon). Cytochemical and ultrastructural investigations were performed for the steroidogenic cells containing 3β-hydroxysteroid dehydrogenase. Five developmental stages were quite distinguishable in M. cephalus males during the testicular cycle: immature stage, stimulating spermatogenesis, rapid spermatogenesis, mature or ripe, and spent stages. The electron microscopy showed that the interstitial (Leydig), lobular bounding, and Sertoli cells inspected during rapid spermatogenesis displayed prominent lipid droplets and cytoplasmic organoids typical of protein-synthesizing cells (i.e. rough endoplasmic reticulum, abundant ribosomes, and distinct Golgi apparatus). In addition, Sertoli cells, in particular, possessed certain microtubules that are known to serve in transporting metabolites. Furthermore, the testes examined after spawning (spent ones) by electron microscope have designated that Sertoli cells, as well as some Leydig cells, had been changed into phagocytic ones to get ride from the non-ejaculated sperm cells. In conclusion, the present results could be very valuable in the problems concerning fish maturation in captivity.