Summary The histological developments of the gonad and the associated sex steroid levels were determined in the breeding stocks of Acipenser schrenckii (age classes 1 to 5) maintained under natural temperature regimes (December 4°C; August 26°C). Early sex differentiation was observed in 1-year-old fish, while testosterone (T) and 17β-estradiol (E2) levels ranged from T 1.1 to 3.4 nmol l−1 (average 1.8 nmol l−1), and E2 varied from 24 to 85 pmol l−1 (av. 50.3 pmol l−1). Gonadal status of 2-year-old males was in stage II while ovaries were at stage I, exhibiting T levels from 1.2 to 4.4 nmol l−1 (av. 2.2 nmol l−1), and E2 concentrations from 10 to 97 pmol l−1 (av. 38.9 pmol l−1). At the age of 3 years, the testes in males were at developmental stage III while the ovaries remained in stage I, with T levels ranging from 1.3 to 21.7 nmol l−1 (av. 9.6 nmol l−1), and E2 concentrations ranging from 17 to 108 pmol l−1 (av. 44.8 pmol l−1). At the age of 4 years, testes in males were at developmental stage III while ovaries in females had reached stage II, with T concentrations ranging from 7.3 to 52.6 nmol l−1 (av. 26.3 nmol l−1), and E2 levels between 13 and 86 pmol l−1 (average 55.3 pmmol l−1). In 5-year-old fish, the testes reached maturity stage while the ovaries were mostly in stage III, with T values from 5.7 to 44.2 nmol l−1 (av. 13.9 nmol l−1), and E2 concentrations from 21 to 453 pmol l−1 (av. 137.7 pmol l−1). Data demonstrated large differences in sex steroid levels among immature Amur sturgeon, and testicular maturation occurred earlier than ovarian maturation.