Abstract: One chick, weighing 1 kg, was inoculated intraveneously with the erythrocytic stages of P. gallinaceum. When the parasitemia reached approximately 40%, a batch of Aedes albopictus, as a control group was fed on the chick. After that, a single dose of Nitroquine (2mg/kg) was given orally to the same chick. 3 hours later, another batch of mosquitoes, as a test group, was fed on again.The samples for ookinete were stained with Giemsa-colophonium method and that for oocyst stained with both Giemsa and Feulgen's methods. The findings are as follows:The average number of ookinetes per midgut in the test group at 24th hour after blood meal was 958.6, that of oocysts at 6th day after infection was 32.7, only accounting for 3.3% of ookinetes. 34 and 48 hours after blood meal, there were a great number of ookinetes adhering to the peritrophic membrane both in the test and control groups. At the same time, 90% ookinetes free in the midgut cavity in the test group appeared shrunk and stiff, the nuclei condensed and black in colour; the cytoplasm dark grayish. However, the majority of that adhering to the peritrophic membrane seemed to be normal in appearence. On the contrary, in the control group there was no visible abnormal change and the average number of oocysts was 361.9 or 65.1% of ookinetes.One of the marked features of the oocysts in the test group was that the quantity of DNA in the course of development was never larger than that of 48 hours oocyst in the control group.All the findings indicate that the sporontocidal effect of Nitroquine may be chiefly to make the ookinetes hard to pass through the peritrophic membrane and to interfere with the reproduction of DNA in oocysts.