The chromosomes of pollem mother cells of the sweet orange (Citrus Sinensis L. ) exhibited translocation,inversion and a synapsis during meiosis after irradiation treatment. These chromosomeal mutations and abortion in the pollen mother cells resulted in decreased fertility and the production of seedless or commercially seedless fruits. Statistical analysis of the data indicated that there was marked inverse relation between the frequency of ceiis with chromosomal mutation and number of seeds in fruit;and marked positive correlation between germinative rate of pollen and number of seeds in fruit. Of all the chromosomal aberrations, the appearance of asynapitc univalent and resultant pollen abortion was the major cause of seedless fruits.