Abstract: The resistance of P.falciparum to chloroquine was assayed by in vitro microtest in 1981 and after cessation of medication in 1984 , 1988- 1990 and 1992 in south Yunnan Province of China.The resistance rates were 97.4% (38/ 39) , 100% (25/ 25) , 96.1% (73/ 76) and 93.7% (59/ 63) ,respectively;ID 50 were 170,132 , 125 and 110 nmol/L ;ID 95 were 1 000,740 , 707 and 576nmol/ L;mean dosages for complete inhibition of schizont formation were 55.4 , 46.7 , 45.8 and 35.4 (pmol/well).A comparison of the results in 1981 , 1984 , 1988- 1990 and 1992 ,the resistance rate showed no marked change,ID 50 dropped 22.3% ( P 0.05) , 26.4% ( P 0.05) and 35.3% ( P 0.01) ;ID 95 declined 26.0% ( P 0.05) , 29.3% ( P 0.05) and 42.2% ( P 0.01) ;mean dosage of inhibition dropped 15.7% ( P 0.05) , 17.3% ( P 0.05) and 36.1% ( P 0.01) . The results show that the extent of resistance in chloroquine resistant P. falciparum to chloroquine in south Yunnan has been declined markedly,indicating that coresistance was variable and dropped along with the declining of drug pressure.