The study was performed with male Russian sturgeon (Acipenser gueldenstaedtii) individuals aged five and seven years and hybrid of Siberian and Russian sturgeon (F1 Acipenser baerii x Acipenser gueldenstaedtii) aged 7 years, reared in a cage farm, located in a warm water reservoir in Bulgaria. Ultrasound monitoring was used to study the development of the testes in the spring – winter period. What was found is that, unlike the Russian sturgeon, in the hybrid, during the vegetation period, the testes changed significantly, increasing all the studied indicators (height, girth and area). In the spring, the highest testicular height was found in seven-year-old Russian sturgeon, and the difference (25%) with the hybrid of the same age was significant (P <0.05). There are significant differences in favor of Russian sturgeon in terms of girth (22%; P <0.001) and testicular area (54%; P <0.001). At the end of the vegetation period, a significant difference (16%; P <0.05) in testicular height was found between Russian sturgeon of different ages, in favor of older fish. There is no significant difference between the different age categories in terms of the girth and area of the testes. Although the differences in the size of the gonads between the Russian sturgeon and the hybrid are not significant, there is a tendency for compensatory development of the gonads in the latter. In spring, the gonads of the hybrid were less developed than those of the Russian sturgeon of both ages, and in winter they were better developed. Our study shows that in an industrial cage farm environment, the male hybrids of Siberian and Russian sturgeon (F1) are not inferior in their sexual development to the Russian sturgeon.