The occurrence of a small number of fruits with empty locules is common for all varieties of Coffea arabica L. and seems to be a physiological phenomenon. Many plants of the Mundo Novo coffee, however, show a high degree of this abnormality. Since this character is undesirable in a commercial variety, a cytological investigation was undertaken to determine its cause. The microsporogenesis in a plant of Mundo Novo coffee with high percentage of empty fruits presented irregularities in the anaphasic distribution of chromosomes ; the same irregularities were found in a plant of the same variety that showed low percentage of empty fruits. The frequency of anaphasic disturbances in both was higher, however, than that in a Bourbon plant. A certain degree of abnormalities was present in the development of the embryo-sac of the same two plants of Mundo Novo. These abnormalities are not commonly found in Bourbon plants. The study of fruits at different ages revealed that in the plant with a high percentage of empty fruits, the increase in the number of these was due to an arrest in the endosperm development at a certain stage, leading to the formation of a small, disc-shaped endosperm. This "discoid endosperm" did not occur in fruits of the Mundo Novo plant that showed low percentage of empty locules, nor in fruits of the Bourbon plants. The development of the endosperm in four other plants of Mundo Novo coffee that showed high frequency of empty locules and in four plants with low frequency of this abnormality was also compared. Again, the "discs" or the "discoid endosperms" were found only in plants with high frequency of empty locules. Also, they accounted for the excess in number of empty locules when plants of the two groups were compared. The present observations indicate that the high number of empty fruits found in certain plants of Mundo Novo coffee is not an increase in frequency of a phenomenon which normally occurs in coffee. It results from an independent and abnormal process that induces an arrest in the endosperm development at a definite stage and reduces it to a characteristic disc shape. It is suggested that the process leading to the formation of disc-shaped endosperm is genetically controlled by a recessive gene d. Plants that show "discoid endosperm" are heterozygous Dd, and the double recessive condition is lethal. Consequently, the development of endosperm of the condition ddd is arrested in the initial stages and a disc is formed in its place. Normal plants are homozygous DD.