Abstract: From September 1985 to May 1987, 30 peoplc were experimentally infected by Ano-pheles sinensis infected with Plasmodium vivax isolated from northwestern part of Hunan Province. The findings displayed in graphs demonstrate that the incubation period was variably depended on the number of infective mosquitoes used in the experiment. The short incubation period of 15.8±1.9d was frequently observed in 6 persons bitten by 10 infected mosquitoes. However, when one mosquito was allowed to fecd on each of 24 volunteers, the incubation period was either short (17.3±1.2d) or prolonged (284.7± 37.7d) in the ratio of l to 1.Based on these data it is suggested that tertian malaria of long incubation period must exist everywhere in China. In addition, the rational classification of vivax malaria and the ratio between long and short incubation periods in nature are also discussed in detail.