Fifty-three cases of falciparum malaria in Linshui County, Baisha County and Sanya Municipality were treated with piperaquine phosphate at a total dose of 1.5g base over 3 days in July to December, 1986. The mean defervescence time was 36 +/- 20.7h; the mean asexual parasite clearance time was 69.7 +/- 20.9h. At 14-28d follow-up recrudescence was observed with asexual parasitemia in 7 of the 47 cases, showing RI resistance to piperaquine. Gametocytemia was positive in 35 cases (74.5%) during the follow-up period.