Early gonad development of the Asian sea bass (Lates calcarifer) grown in a freshwater recirculation system was investigated to determine the onset of sexual maturation. Studied individuals aged between 9 and 15 months. Culture conditions were provided initially in circular tanks (4000 L) and linked to a warm-water recirculation system (27°C, O2 = 7–8 mg L−1) until they reached 20 g wet weight. They were ongrown in raceways (54 m3; final density 80 kg m−3) connected to a recirculating system with a fluidized bed as biofilter. Their gonads were examined each month (5–12 fish) and prepared for histology. Following microscopic observations testis maturity was categorized and correlated to total length (cm) and weight (g). Gonad development started much earlier in the recirculation system than described previously in both, wild and cultured fish. We assume that spermatogenesis in these fish starts earlier than 9 month of age and the fact that mature spermatozoa were present in testes that contained mostly spermatogonia indicates highly asynchronous spermatogenesis.