This paper describes experiments made to develop a technique for marking insects and get the informations for ecological studies as a means of tagging flies with radioactive phosphorus, and also to learn the effect on the subsequent generation of flies. Tests were conducted with house flies, Musca domestica L. emerged front the larvae that had reared in a media containing various concentrations of P-32. Radioactivity of the flies was measured with end-window type $(2mg/cm^2)$ Geiger-Muller counter and proportional counter made by Aloka. In following the daily increases in counting rates by 6-day-old larvae it was apparent the larvae in the starting media containing the higher concentrations of P-32 were showing high counting rates. It was found that about 13.2 per cent of the radioactive material remained in the empty puparia after the flies emerged. Radioactivity of the emerged flies decreased gradually and the biological halflife of phosphorus-32 was approximately 6.14 days with considerable variation. A significant amount of P-32 were detected in the abdominal and thoracic sections as compared with other parts of adult. The activity of phosphorus·32 in eggs and larvae produced by adults reared in medium containing 0.2 microcurie or more of P-32 per gram were readily detected with Geiger-Mi·lier counter. On the other hand, the radioactivity of eggs and larvae produced by the treated males crossed with untreated normal females could be only detected in the medium containing more of 2.0 microcurie with G.M. counter. There was no apparent damage to the flies at dosages as high as 0.5 microcurie per gram of medium. For field studies a concentration of 0.5 microcurie of P-32 per gram of artificial medium appears to give adequate labeling for positive identification of flies and their egg masses.