Two hundred and fifteen individuals of the tiny abyssal scallop (Delectopecten vitreus macrocheiricolus) were collected at a depth of around 3,000 m on the bottom of the Japan Sea during a cruise of the Yokosuka/Shinkai 6500 YK 01-06. The scallop population included specimens of 4 sizes, i.e., very small, small, middle, and large, with mantle sizes of 1–2mm, 3–5mm, 6–9mm, and 10–12mm, respectively. Histological observations revealed that the large-size group was at the stage just after oviposition or spermiation, or in the regressed stage of gametogenesis. The small- and middle-size groups were in the growing stage of gametogenesis. These results suggest that only the large-size individuals were responsible for reproduction and that the small- and middle-size individuals would participate in subsequent reproductions. This paper will discuss the reproductive strategy of this scallop living at such great depth.