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ABSTRACT The gonadosomatic index (GSI), ratio of area of the lumen to that of the seminiferous tubules, and sperm density were analyzed against body size and season in male spiny lobsters, Panulirus japonicus, sampled off Oshima Island, Tokyo, Japan. For functionally mature individuals (≥54 mm carapace length), regressions of GSI and area of the lumen against body size were significant only before the spawning season. GSI showed no clear seasonal changes. Germinal cells at all stages of spermatogenesis were found in the testis, and sperm were present in the vas deferens throughout the year. However, seasonal changes in the 3 reproductive indices indicate that P. japonicus has a distinct seasonal pattern for spermatogenesis and storage of sperm. The ratio of the area of the lumen to that of the seminiferous tubules was highest before the spawning season. Sperm density during and 1 month after the spawning season (from June to September), however, was significantly lower than in other months.