In an attempt to the effect of continuous backcrossing to M flies on the P element activity of males in Drosophila melanogaser, P flies and their offspring were backcrossed with M flies. Two components of dysgenesis, P element activity and P element DNA amount, were examined during the six generations. From the result of experiments where P flies were backcrossed to M flies, the ability of males to induce gonadal dysgenesis decreased rapidly until the 6th generation where it was below 5% in the dysgenic crosses and the similar results was detected in the nondysgenic crosses. The amount of P elements DNA was measured by Southern analysis using blotin-labeled pπ25.1 probe. The total amount of autonomous P element DNA were decreased gradually. It was suggested that the ability of flies to either induce or suppress gonadal sterility in different generations seems closely correlated with the amount of P element DNA. these results seem to support in stochastic loss hypothesis of P element in the natural populations of Drosophila.