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We sampled cultured juveniles of the ayu Plecoglossus altivelis that would be used for river stocking, to evaluate histologically the development of the organs that are important for immunological competence of the fish. These samples included hatchery-produced fish reared in three different facilities (sample A), fish captured in Lake Biwa when they were larvae and cultured by three different fish farmers (B), and wild juveniles as the control. Each sample consisted of at least 10 fish. Histological observation and following quantification showed clear positive correlation of the thymus volume and the body length of the wild fish. However, the fish in each 2 groups of the samples A and B had significantly smaller thymuses than those of the wild fish of corresponding body lengths. Congestive splenomegaly was often observed in the cultured fish for both groups. To know the rearing conditions that affect thymus development, a preliminary experiment was conducted in which larval ayu were reared in aquaria for 3 months in four different rearing conditions with the combinations of two different water temperatures (13.8 and 17.2 °C) and two different stocking densities (0.25 and 2.5 kg/m3 at the start of the experiment). The result showed that both the high water temperature and the high rearing density suppressed development of the thymus significantly, whereas the fish reared in the combination of low water temperature and low rearing density developed the thymuses of the comparable size to those of the wild fish. These results suggest that inappropriate culture conditions may suppress the thymus development, even if the fish appear to grow normally. [Copyright &y& Elsevier]