For Dobzhansky''s (1935) suggestion, by way of explaining the case of D. miranda, in which an X-0 type of sex-determining mechanism is superimposed upon an X-Y type, namely, that both types are here effective, the author offers an alternative explanation as follows. The X1-Y pair of chromosomes is effectively homozygous with regard to sex, containing a middle pairing segment derived from that of the X-Y pair of the other species, its arms derived partly from auto-somes and partly from sex-indifferent parts of X and Y. The pairing segments still show the special pairing mechanism, by reciprocal chiasmata found in X and Y of the other species, and by a failure of pairing in the cells of the salivary glands. A displaced mechanism here survives its use. The X2-chromosome, on the other hand, contains the sex-differential part of the X-chro-mosome of the other species. The essential sex-determining mechanism is, therefore, of the X-0 type.