The structure and development of mucilage-crystal idioblasts in the roots of five Actinidia species, A. hemsleyana, A. eriantha, A. rufa, A. deliciosa, and A. chinensis, are comparable. Idioblasts are present in the cortex of primary roots and in both the cortex and phloem of secondary thickened roots. Idioblasts are first visible immediately behind the root apex. Young idioblasts are characterized by an absence of starch grains, large amounts of ER, and dense cytoplasm relative to that of adjacent parenchyma. Raphide crystals are formed in individual vacuoles at an early stage in idioblast development. These small vacuoles subsequently fuse into the central vacuole, and mucilage accumulates around raphide bundles. As idioblasts enlarge, they accumulate more mucilage. Mucilage exhibits variable numbers of strata, which stain differentially with toluidine blue O. Large, empty cells observed in outer regions of phloem in substantially thickened roots may represent a late stage in idioblast development.