AbstractMelanin granules of rhesus monkey eyes develop in four stages. Early in stage I, they are small, spherical or elongated, membrane‐limited vesicles containing parallel arrays of membranes oriented along the long axis and having regular periodicity. Melanization of these membranes, which is indicated by increased thickness and electron opacity, leads to the formation of stages II and III granules. In stage IV, their internal structure is completely obliterated by electron‐dense melanin. During this process of development, melanosomes are transformed from thin, long vesicles into large ovoid bodies.By 60 days of gestation, melanosomes have developed in the pigmented epithelium of the iris, ciliary body, and retina, those in the retina being more mature and fully pigmented than the others.Stromal pigment cells of the uveal tract develop later than those of the pigmented epithelium. Pigment first appears in the iris at 140 days, then in the ciliary body, and lastly (150 days) in the choroid, where only well‐developed melanosomes are found.Melanosomes of the stromal pigment cells as well as those of the pigmented epithelium may have their origin in the iris. Early melanosomes (stage II) are present almost exclusively in the iridial cells whereas mature forms are found in the ciliary and retinal cells.