A study on the genetic behavior of sexually disadvantage between sex-linked mutants and the wild type of Drosophila melanogaster collected at Keojaedo was carried out. For the present experiment using population cages, each fly of two cages reared under the influence of two different strength of magnetic fields, i.e. the without magnetic field generated by Helmholtz coils and the ambient magnetic field. The mean selective frequencies of gene-carrying chromosomes of wild type up to twentieth generation, affected by two different magnetic fields, showed 0.7212 and 0.7180 with the magnetic field strength as previous mention. The data explains that Drosophila melanogaster population reared in the without magnetic field showed lower selective disadvantage. The selective frequencies of each individual of yw gene-bearing chromosomes showed 0.0306 and 0.0372 according of magnetic field strength indicated above. It was shown that the fly population reared at the ambient magnetic field cleared the relatively lower selective disadvantage. The over-all mean emergence rates of Drosophila melanogaster populations were 53.40% and 46.60% respectively in controlled magnetic fields. The fly population raised in the ambient magnetic field showed a relatively lower emergence rate than the without magnetic field population. Through Duncan's multiple range test and LSD value, the highly significant differences between the wild type and anyone of the other three mutants were calculated.