Abstract: This paper deals with some biological and clinical aspects of Plasmodium vivax in Yunnan and Hunan Provinces. The results obtained are as follows: (1) The developmen-tal period of P. vivax in mosquitoes was 9 days at a temperature of 26±1℃ and the parasitaemia was from 8 to 13 (9.9 ±1.3) days after exposure in Yunnan, 11 to 16 (14.3±1.6) days after exposure in Hunan. No pronounced correlation bctween the length of prepatent period and the number of sporozoites incculated was found. (2) In most of the cases artificially infected with Plasmodium vivax the clinical attack occurred in the afternoon and the typical tertian fever was seen only in 2 of 14 patients. The natural course of infection was as short as 9 to 14 days in nonimmune cases (Fig. l, 2). (3) Mosquitoes fed at the time when parasite was first detected after sporozoite inoculation did not become infected. But the infection rate of mosquitoes fed on patients at the first onset was 7.9% in Yunnan and 93.0% in Hunan. Such a grcat difference in infectivity was briefly discussed in terms of parasite biology.