A young flower stalk of pin-formed (pin) mutant of Arabidopsis thaliana forms no floral organ and bares its growing apex. In the latter stages of growth the apex of the flower stalk becomes fasciate or circular, and then develops numerous deformed flowers from its flanks. The flattening of the apex and the variety of flower morphology are more remarkable in the plants carried over winter in a greenhouse than those grown in a growth chamber of controlled temperature and light. The flowers of the pin mutant are entirely sterile, and the developed floral organs (sepal, petal, stamen and pistil) show various degrees of abnormality in number and shape. Among floral organs carpels are most predominantly formed, and the carpels often develop into pistils with ovary, stigma, papillae and ovule-like particles. Various homeotic transformations such as sepal-to-carpel, petal-to-carpel and stamen-to-carpel occur. The preferential carpel formation strongly suggests an epistatic relation between carpel gene(s) and genes controlling other floral organs, i.e., expression of carpel gene(s) is indispensable for differentiation of the four floral organs. If the genes controlling the development of sepals, petals or stamens would not function, carpeloid organs would inevitably appear on their whorls. The hierarchical relationship among the genes controlling the floral organ development is discussed.