Testes of the early and middle third larval instar of D. hydei were cultured for a period of 1 to 8 weeks in adult hosts of both sexes. During the culture period spermatogonia and primary spermatocytes passed through the meiotic divisions and differentiated into late elongated spermatids. The latter could be observed with still normal ultrastructure, but also in different stages of degeneration.Autoradiographic studies revealed a normal time course of spermatogenesis during the first week of culture in the adult host; further culture led to a retardation in development. Measurements of the teste's length and autoradiographic labelling showed that the gonads grew only slightly while cultured in the adult host. The size of the testes and their spermatogenic activity are correlated such that the number of postmeiotic germ cell cysts increases with the volume of the gonad. Spermatogenesis is more intensive in tested which were cultured in females than in those cultured in males, and is also favoured in metamorphosing hosts as compared to adult flies. Optimal nutritive conditions of the host flies evoked an increase in the testis' length and number of postmeiotic cysts formed, compared to those which were simply reared on standard food. The addition of exogneous β-ecdysone stimulated spermatogenesis and somatic growth, especially at suboptimal nutritive conditions. It is suggested that both nutritive conditions and the level of ecdysone play a role in gonadal development.