This study devised and tested a histological staging system for gonadal development in the trumpet snail, Charonia sauliae, which was collected along the south coast of Jeju Island, South Korea. This paper details for the first time the gonadal development characteristics of C. sauliae. Ovary and testis development in C. sauliae can be roughly divided into five stages: growing, mature, spent, degenerative, and resting. The trumpet snail has multiple fecundation and fertilization [cycles?] during the spawning season for out-of-step gonadal development in high-temperature and low-salinity environments. Female trumpet snails tended to have larger shells and were more abundant than males (64.26% of all animals collected were female).