Abstract: From July 1983 to September 1984, experimental observations on healthy volunteers were carried out in the tropical and sub-tropical regions of Yunnan Province to determine the biological characteristics of Plasmodium vivax including prepatent period, incubation period, relapse potential and sensitivity to primaquine.39 persons devided into 5 groups were exposed to 1 to 10 mosquitoes infected with Plasmodium vivax obtained from different parts of south Yunnan Province. Among them 37 showed a prepatent period of 8-13 days, the average being 10.0±1.3 days, an incubation period of 12.2±1.9 days with a range of 10 to 17 days; 2 showed a long incubation period of 268 and 284 days, respectively.The relapse rate after treatment with chloroquine 1. 5g (base) alone or chloroquine combined with 14 days of primaquine was 95.2% and 0, respectively. In 75% of the participants the first relapse occurred after 228.8±45.6 days, followed by one or more subsequent relapses with much shorter intervening period of latency, most often ranging from 6 to 9 weeks. In 10% the first latent period was short, being 72±20 days, followed by a long second latent period (136±12 days). These relapse patterns are similar to that classified as the "temperate zone" type. In the second group which had been subdivided into two subgroups of 3 patients each the features of either Type I or II were observed. Namely, the pattern in the first subgroup had the characteristics of the Chesson-like type with no long period of latency between primary attack and subsequent relapses, and in the second subgroup the pattern presented the characteristics of temperate zone, which had a long period of first latency.On the basis of their life patterns, it could be concluded that: (1) The long incuba tion period is not confined to temperate zone strain; (2) The vivax malaria observed in the study areas briefly fell into the "temperate zone" category.