The sensitivity of Plasmodium falciparum to chloroquine in vivo was examined out in a small town in the Red River basin from 1995 to 1996. forty cases were recruited into the present study, 32 cases were completely followed-up. The results of the in vivo study revealed that only 3 (9%) cases were sensitive to chloroquine, 29 (90.6%) showed resistance to chloroquine, among whom 7 cases (22%) showed resistance at RI, 12 cases (38%) at RII, and 10 cases (31%) at RIII. It is suggested that P. falciparum endemic areas should stop using chloroquine and other 4-aminoquinolines in the Red River basin now. Qinhaosu and pyronaridine were recommended to use as the first line antimalarial drugs of P. falciparum infection in the basin.