A study of the blood parasitism by Leucocytozoon in chickens by breed in the province of Thai Nguyen indicated that: By the clinical signs, the prevalence of the infection was found varying from 15.22 to 19.59 percent depending on the chicken breed. Precisely, the prevalence in the Mia chicken flocks was 35.48 percent (11/31 flocks)
in the Luong Phuong's one: 37.84 percent (14/37 flocks)
in the Sasso: 40.91 percent (9/22/flocks) and in the native chickens: 38.46 percent (15/39 flocks). By the hematological examination, the parasites were found in both the sick and the healthy chickens. The positiveness was found in 84.05 percent of the clinically sick and in 35.87 percent of the apparently healthy chickens. The intensity of the infection was found high in 52.55 percent and very high in 30.66 percent of the sick birds: the intensity was qualified as lightly in 18.18 percent and very lightfy in 81.82 percent of the healthy chickens.