The wild Modiolus modiolus resource has experienced a dramatic decline in abundance over the past few decades because of overfishing, declining seawater quality, and a lack of proper protection. The reproductive cycle and gonadal development of M. modiolus from the coast of Heishijiao, Dalian city were investigated from October 2013 to September 2014 to develop management strategies and optimize hatchery-based seed production. Specimens were col- lected monthly, and seawater column temperatures were recorded simultaneously. Shell size, total weight, tissue weight, and gonad weight of M. modiolus were measured to calculate fatness and the gonadosomatic index, and histological techniques were used to analyze the seasonal gonadal cycle. The histological analysis showed a unimodal gametogenic cycle with one spawning season (August–October) annually. Although shellfish gonadal development stages have no unified standard, M. modiolus gonadal development was divided into five stages according to previous studies: the pro- liferating stage, growing stage, maturing stage, spawning stage, and resting stage. Oogenesis was divided into the oogo- nial stage, early growth stage, middle growth stage, late growth stage, and mature stage, according to the oocyte devel- opmental characteristics and the proportions of different oocyte developmental stages in the follicle. Spermatogenesis of M. modiolus included five stages of spermatogonium, primary spermatocyte, secondary spermatocyte, spermatid, and spermatozoon. Spermatogonia were morphologically divided into types A and B. Type A spermatogonia distributed in all developmental stages, and their nuclear chromatin was arranged loosely. Type B spermatogonia occurred during the proliferating stage, and the nuclear chromatin was concentrated. Thus, types A and B spermatogonia were considered differentiated cells and stem cells, respectively. The peak gonadosomatic index of 32.25% and fatness of 38.43% were observed in January, and the maximum values for both occurred in August. The breeding season was from June to No- vember, when the average seawater temperature was 16.8–22.1℃. M. modiolus gonads matured by the end of May, but they did not spawn until August of the rainy season, indicating that the mature gonads of the parental shellfish only pro- vided a material basis for reproduction and that the reproductive process requires particular conditions for induction, such as temperature and salinity. The developmental rate of the male gonad was slightly faster than that of the female. Gonochorism existed, and the female to male ratio was 1︰1. These results provide basic information about M. modio- lus reproduction to help with natural resources protection, and will aid in developing a large-scale artificial breeding program.