Effects of rearing density on growth and maturation of 0-year-old Cottus kazika were studied over 4 years. Juveniles 90 days after hatching were used in the following 2 rearing conditions: 1) 52 fish were reared in 150 liter tank, representing the high-density condition, 2) each of 52 fish were reared in individual 57 liter glass aquaria, representing the solitary condition. These experiments were continued until 300 days after hatching. Almost of all fish were mature in the high-density condition. Whereas, large sized fish were mature and small sized fish were immature in the solitary condition. Therefore, data over the 4 years demonstrated that the maturation of 0-year-old C. kazika was quickened under the high-density condition.