- While molting and reproduction in female crustaceans are often known to be coupled processes, the explicit nature of this coupling has not been studied in males. We examined the relationship between molting and spermatogenesis in 2 morphotypes of the freshwater prawn, Macrobrachium rosenbergii. These morphotypes were the rarely molting, dominant, blue-claw male and the frequently molting, subdominant, orange-claw male. Testes from the 2 morphotypes were dissected and cultured in vitro. The rate of [3H]-thymidine incorporation (as a measure of DNA synthesis) was recorded. Autoradiography was performed with histological sections of the cultured tissue. A strong correlation between the incorporation rate and the molt stage was found. The blue-claw males were intermolt stage C4 and the testicular [3H]-thymidine incorporation rate was relatively low. The incorporation rate in premolt orange-claw males was at least three times higher than in these results: heavy silver grain precipitation was observed on top of the spermatogenic zone in the testicular sections taken from premolt orange-claw males. The correlation between DNA synthesis in the testes and the molt stage suggested that ecdysteroids either directly or indirectly participate in the regulation of spermatogenesis. We observed that 20-hydroxyecdysone did indeed have a direct effect on DNA synthesis in cultured testis. Addition of 2.6 × 10−6M 20-hydroxyecdysone increased [3H]-thymidine incorporation into testes of premolt prawn by more than 50%.