Reproduction is one of the most important processes in the life history of fish. However, there is little information about the reproductive biology of the fourfinger threadfin, (Show, 1804). In this paper, paraffin tissue sections were used to observe the developmental process of testes via haematoxylin-eosin (HE) staining and transmission electron microscopy (TEM) in order to understand the organizational structure of the testes as well as the histological and ultrastructural changes associated with spermatogenesis during primary sexual maturity in cultured fourfinger threadfin individuals. The results show that the testes are a pair of extended flat zonal organs that are gray in color and fuse at the posterior end into a Y-shape. The organs are located on the dorsal side of the abdominal cavity, attached to the ventral surface of the mesonephros and swim bladder. The developmental process of the testes was divided into six stages defined by histological characteristics. The testes develop to stage I (spermatogonia proliferates), followed by stage II (spermatocyte growth), stage III (spermatocytes mature), and stage IV (sperms begin to appear) at about three months of age, four months of age, five to seven months of age, and seven to nine months of age, respectively. At a minimum of 10 months of age, the testes develop to stage V (complete maturation of sperm) and reach the first stage of sexual maturity. The post-reproductive testes in stage VI (degradation and absorption) were observable after the breeding season. Spermatogenesis went through six phases, namely primary spermatogonia, secondary spermatogonia, primary spermatocyte, secondary spermatocyte, spermatocyte, and sperm in succession. The diameter of cells and nuclei decreased step by step, and the ratio of nucleus to cytoplasm changed regularly. During spermatogenesis, chromosomes in the nucleus were gradually condensed, electron density was increased, and mitochondria were enlarged and aggregated with inner cristae structure gradually enriching. Sperm was composed of a head, middle, and tail. The axial filament of the flagella followed a typical "9+2" structure. This study elaborated on the organizational structural characteristics of the testes and the histological and ultrastructural changes during testes development and the spermatogenesis process of first sexual maturity stage of cultured fourfinger threadfin. This information enriches our collective knowledge on the species' reproductive biology and provides a theoretical reference for understanding reproduction laws and improving artificial breeding technologies.