The freshwater prawn Macrobrachium rosenbergii shows three male morphotypes: blue-claw males (final stage having high mating activity), orange-claw males (transitional stage showing rapid somatic growth), and small males (primary stage showing sneak copulation). This morphotypic differentiation is considered to be controlled by androgenic gland hormone, which is probably a peptide hormone. However, its physiological roles are not fully understood. In the present study, we examined the correlation of androgenic gland cell structure to spermatogenic activity and morphotypic differentiation histologically in M. rosenbergii. Spermatogenic activity showed close correlation to the molt cycle in orange-claw males and small males. Spermatogonia increased in number in the late premolt stage, becoming spermatocytes in the postmolt stage, and spermatocytes differentiated into spermatozoa in the intermolt and early premolt stages. Ultrastructure of the androgenic gland was additionally compared among the molt stages, but, distinct histological changes were not observed in relation to spermatogenesis during the molt cycle. On the other hand, among the three morphotypes, the androgenic gland was largest in the blue-claw males, containing developed rough endoplasmic reticulum in the cytoplasm. These results suggest that, during spermatogenesis which is related to the molt cycle, the androgenic gland hormone is at rather constant levels and plays a role in maintaining spermatogenesis rather than directly regulating the onset of a specific spermatogenesis stage and that, during the morphotypic differentiation, the androgenic gland is most active in the blue-claw males and plays a role in regulating the observed high mating activity in M. rosenbergii.