Females heterozygous for a wildtype X chromosome from a Samarkand stock and an chromosome of the composition [formula] were tested for the frequency of X-chromosomal non-disjunction and segregation of the X's when Y chromosomes of different types were present in the females. The Y chromosomes used were 1) a normal unmarked Y, 2) [formula], 3) [formula] and 4) [formula] (a chromosome which arose in one of our experiments and has not yet been analyzed). Since this was only a preliminary test no attempt was made to isogenize the stocks. Larger scale experiments are planned in which these and other Y's will be used and the genetic background will be strictly controlled.