Hilsa shad (Tenualosa ilisha) is an anadromous species which has reproductive migration from the Persian Gulf to the river. Morphological properties and histological changes in oocyte of Hilsa shad in two different environments (salt water and fresh water in Khuzestan province) were studied with respect to its reproductive cycle. Twenty fish samples with average weight of 830.75±3.25 gr, were collected from Bahmanshir (freshwater) and Musa creek (estuarine water). Following biometry (length and weight measurements), pieces of ovary of each fish were fixed in Bouin's fixative solution. Microscopic sections (5μm thickness) from paraffin embedded tissues were prepared and stained with Hematoxylin & Eosin, and then slides were studied under light microscopy. Results showed six phases as follows: chromatin nucleolus, perinucleolus, cortical alveolus, primary vitellogenesis, secondary vitellogenesis and third vitellogenesis were recognized in ovaries of T. ilisha. Three follicular layers such as theca, granulosa and zona radiata were clearly observed at the end of the vitellogenesis. The most oocyte diameters were also measured at the same phase. Ovarian follicles were at the same phase as in the ripe ovaries, indicating Hilsa shad is synchronous fish species in ovarian development. The changes observed in the pattern of oocyte development in T. ilisha was typical of the patterns described in most synchronous spawners studied to date.
