Very few studies on developmental histology of thymuses at different gestational ages were reported in literature. In the present study the developmental histology of thymus in human fetuses of different gestational ages were observed. MATERIALS AND METHODS: A total of 30 thymuses of 6 weeks gestational age to full term collected from aborted fetuses, were processed for routine paraffin embedding, sectioning and staining with Haematoxylin & Eosin stains. The histological sections were observed under binocular research microscope for age related changes in microscopic structure of the human thymus and representative fields were photographed. RESULTS:The earliest specimen of 6 wks gestational age presented the thymus along with parathyroid as ventral primordium of 3 rd pharyngeal pouch. Cortex-medulla differentiation and early stage of formation of Hassall's corpuscles were observed at 14 -16 wks. At 36 wks "starry-sky" appearance, an indication of emperipolesis was observed at the cortico-medullary junction. Hyalinization of Hassall's corpuscles, their keratinization and high endothelial venules were seen at full term. CONCLUSION: There is a delay of 5 weeks in cortico-medullary differentiation and a delay of 3wks in the time of appearance of Hassall' corpuscles in the present study when compared to the reported studies in literature.