In order to investigate the chromosomal effects for cytotype determination, two Q one M’, and Canton S(control) Strains were selected from Korean natural population and laboratory of Drosophila melanogaster. From the matings of the above straind with `SKK` strain (i.g., Basc; In (2LR) SM1, Cy/In(2LR) bw^(VI) : In(3LR) TM3, Ser/Sb), twelve chromosome lines per strain were established under M or native cytotypes. Thereafter, the cytotypic change was checked with generations and the relative magnitude of chromosomal effects on the changes were examined. The results are as follows: Until the 12 generations, any cytotypic change was never detected in the lines derived from Canton S. In the lines from M’ strain, no changes to P cytotype were detected, except one case of a combination of X and 3rd chromosomes. But among the lines originated from Q strains, the cytotypes were found to be primarily determined by the genotype after only one or two generations, and to be subordinately affected by a maternal cytoplasm in some lines. In one Q strain, major cytotype determinations were detected in the lines including a wild X chromosome, and also detected in a combination of 2nd and 3rd chromosomes excluding a wild X chromosome. This figure are not consistent with the another Q strain, in which apparent change of to P cytotype was detected in only one combination of 2nd and 3rd chromosomes. Thus, this finding indicates that the cytotype determination in Q strains is dependent on chromosomal interaction and/or a particular chromosomal factor. The further molecular analysis will be provide a evidence on whether the above chromosomal effects in cytotype determination of Q strain are due to mainly by an interaction of P factors and/or KP elements in genome, by an absolute quantity of KP elements, or by a joint effect with another defective P elements between chromosomes.