Corolla tube formation was investigated anatomically for 13 species of the Primulaceae and Ericales. In the Primulaceae, petal primordia appear after the stamen initiation. The lower portion of the corolla tube, the portion under the stamen insertion, is derived from the ring-like structure formed immediately after the stamen initiation. Petal bases extend laterally, and connect mutually on the ring-like structure. The upward growth at the connected regions results in the formation of a corolla tube, which develops into the upper portion of the corolla tube. In the Ericales, except forClethra, the corolla tube is formed by the mutual connection of petal bases. This tube elongates inRhododendron, Leucothoe andPieris. However, inPyrola, Ledum andTripetaleia, the elongation of this tube is meager, resulting in a dialypetalous condition in the mature state. Three petals ofTripetaleia may be derived from the partial connection of five petal primordia in a very early stage of the corolla development. From the data obtained in the studies on the corolla tube formation, three modes of the corolla tube formation are suggested; postgenital fusion, connection of petal bases, elongation of the ring-like common base.