M and M′ strains of Drosophila melanogaster in the P-M system of hybrid dysgenesis were compared in two series of tests, with the following results. (1) The singed-weak hypermutability regulation test showed that M′ strains had lower P excision rates than M strains, suggesting that P-elements repression must occur in M′ strains although it is not detectable by gonadal dysgenesis assays. (2) The evolution of mixed P + M and mixed P + M′ populations was compared, using a strong P strain. The P + M cultures invariably evolved in a few generations into strong P cultures, while the P + M′ cultures evolved into P-type cultures with reduced P-factor potentials. However, after 30 generations of culture, both these types of mixed cultures had similar P copy numbers, suggesting that regulation of copy number had occurred in them.