1.XXY females heterozygous for theClB chromosome give among their offspring about 35 per cent. of secondary exceptions. The percentage of secondary exceptions is equal in the male and female offspring. TheY-chromosome disjoins from theClB chromosome and from the normalX-chromosome with equal frequency. 2. InClB females crossing-over between theX-chromosomes is greatly inhibited. The percentage of cross-overX-chromosomes carried by the gametes ofXX-ClB females is equal to that ofXXY-ClB females, indicating the correctness of Anderson’s conception of the mechanism of secondary non-disjunction. 3.XX-ClB females give among their offspring about 0·35 per cent. of primary exceptions. The comparison of the number of non-cross-overX-chromosomes produced by such females, with the frequency of non-disjunction in their offspring shows that the mechanism of chromosome disjunction in the male and femaleDrosophila is essentially similar, and speaks against Darlington’s views concerning the relation between crossing-over and the disjunction of chromosomes.