Studies on the ultrastructural aspects of spermatogenesis and, specifically, the structure of sperm in aquatic Heteroptera are scarce. Therefore, the objective of this study was to analyse of the histology and ultrastructure of spermatogenesis. Semi-fine sections of the testicles of adult male Gelastocoris flavus flavus were stained with toluidine blue or impregnated with silver ions, and ultra-fine sections were analysed by transmission electron microscopy. The ultrastructural features observed during spermatogenesis of the species showed the presence of several small mitochondria uniformly distributed in the cytoplasm of cells in prophase I. These mitochondria then came together to form fewer, larger structures, which converged and formed the mitochondrial complex. Later, this mitochondrial complex was divided into two structures, termed mitochondrial derivatives, which were arranged bilaterally to the axoneme. The axoneme showed a flagellar pattern of 9+9+2. A vesicle was observed that originated in the early stages of spermiogenesis and was composed of many argyrophilic granules that united to form a single structure. This vesicle contained some highly stained structures in its interior. Thus, this paper describes histological and ultrastructural characteristics during spermatogenesis, contributing to the reproductive knowledge of these aquatic Heteroptera.