The morphology and development of the granules of developing eosinophils present in the trunk kidneys of Rana esculenta tadpoles has been studied at the ultrastructural level. During the differentiation of eosinophil granulocytes a single and morphologically unique population of cytoplasmic granules is present. Fully developed granules are spherical, membrane-limited, and have a dense and homogeneous content. Eosinophil granules have been traced back to Golgi-derived vacuoles which fuse and form immature granules containing a dense and knot-like precipitate. By condensation these immature granules transform into the definitive homogeneous forms. The eosinophil granules of Rana esculenta are compared to those in other vertebrates and possible phylogenetic aspects are discussed.