Abstract: The Fl hybrid plant between Oryza sativa L. and Pennisetum sp. was obtained by Teng Yen Tang in 1958. In many morphological characters the hybrid appears to be superior to the parental plants, such as the culms are taller, the leaves are comparatively longer and broader, and the spikes are apparently much larger. However, the form of the spikes of the hybrid plant is spreading type which resembles that of the mother plant. In general, the hybrid plant bears a close resemblance to the female parent, though its spikelets produce conspicuous long awns which must be inherited from the pollen plant. The Fl hybrid is almost completely sterile, for it only occasionally produces one or two grains in open condition. Owing to the high regenerating feature, the hybrid plant has been successfully cultivated by vegetative propagation. In a majority of cases, megaspores have been observed to degenerate in rather early stages. More than half of the ovaries examined thus lack embryo-sac formation, they seem to grow parthenocarpically to some extent before degeneration sets in. Embryo- sacs have also been observed to degenerate sooner or later after their formation. In a well formed embryo-sac, when it does occur, the normal three successive mitoses of the nucleus of the embryosac mother cell do not occur regularly, and the matured embryo- sac which results, lacks some of its apparatuses. In. some cases, the apparatuses differentiate in abnormal way. The number of the polar nuclei may be more than two, that of the antipodals is, sometimes, less than three, and the egg apparatus may be composed of four cells instead of three. It is apparently that the apparatuses of the embryo-sac produced in such abnormal manner are functionless. In other cases, the sac loculus of an enlarged ovary has been found to contain nothing but liquid. Only in very rare cases, the formation of the embryo-sac and the differentiation of the apparatuses have been carried through in the proper manner. The chromosome number of Oryza sativa L. is n = 12 and that of Pennisetum sp. is n=9. Both the somatic and the gametic chromosome numbers of the Fx hybrid are variable and they generally do not correspond to the sum of the reduced chromosome numbers of the parents, unlike what they usually do in the intergeneric hybrids raised from other cereals as reported by many investigators. In meiosis of the hybrid, the chromosome behavior is very irregular, different numbers of uivilents and bivalents have been counted in different pollen mother cells, and one or two trivalents may also be present in some of the pollen mother cells. The trivalents are often V-shaped, but some- times they are chain-shaped. A number of lagging chromosomes have been met with at metaphases and anaphases of both first and second meiotic divisions. During the formation of the restitution nuclei the laggards are often excluded, but they do not form micronuclei. In both heterotypic and homotypic meiotic divisions the spindle formation is regular. There is no unequal division of the pollen mother cell. Multipolar spindle and multispindle are not present, and finally normal tetrads result. However, the tetrads do not develop further and most of them soon degenerate. There is only a small number of floulets in the anthers of which a few fertile pollens have been observed. It is evident that all the irregularities occurred in the course of embryo-sac formation and differentiation together with the breakdown of the meiotic cycle in the pollen mother cells are the causes of the sterility of the Ft hybrid plant.