The investigations were carried out about the reinfection with leucocytozoon disease in yearling hens that were infected with this disease in the preceding year. From the investigations carried out in 1962, 1963 and 1964, the following results were obtained. 1. Among the 20 hens once infected with this parasite, the reinfection was not at all found in the same and the following years by the microscopic examination of blood smear sample made once every week. 2. Among the 22 yearling hens that were completely protected from leucocytozoon infection in the preceding year by the use of pyrimethamine or sulfadimethoxine, it was found by the blood examination that 11 hens (50%) were infected with this disease in the following year. 3. Eleven hens, in the blood of which no gametocyte had been detected in spite of the fact that they had been among the highly infected flock, were not at all infected in the following year. From the results mentioned above, we have come to the following conclusions. Hens recovered from natural infection of leucocytozoon disease acquire the strong immunity later. And the effect of pyrimethamine and sulfadimethoxine used in these experiments are quite perfect.