Female sea urchins store the major yolk protein (MYP) in ovarian nutritive phagocytes before vitellogenesis. Using immunological procedures, we detected MYP in the testicular nutritive phagocytes of Pseudocentrotus depressus, the red sea urchin, and then compared the distribution of MYP between sexes during gametogenesis. MYP was purified from unfertilized eggs by ion exchange chromatography (Q Sepharose) and gel filtration (Superdex 200), and an antiserum (anti-MYP) was raised against MYP. Immunoblot analysis demonstrated that immature testes, as well as ovaries, contained a large quantity of MYP. Immunohistochemistry showed that MYP was distributed in the nutritive phagocytes occupying the follicular lumen in both males and females. In both sexes, as gametogenesis proceeded, the nutritive phagocytes degenerated and the gonadal lumen filled with gametes. MYP accumulated in ripe ova as a yolk protein in the mature ovary. In contrast, MYP was not detected in mature testes, because stored spermatozoa did not react with anti-MYP. We conclude that in male P. depressus, MYP is stored in the testicular nutritive phagocytes and utilized as the nutrient source for spermatogenesis.