Abstract: The sensitivity of P.falciparum to mefloquine,quinine,amodiaquine,chloroquine,pyronaridine,and sulfadoxine/pyrimethamine(MEF,QNN,AMO,CHL,PYR,S/P)was tested using in vitro microtest in south Yunnan Province in 1992.The ID 50 were 46,480,52,150,15 and 2×10 4/25×10 2 nmol/L,respectively;ID 95 were 160,1 536,292,680,60 and 24×10 5/ 3× 10 5 nmol/L,respectively.All the isolates(30) were sensitive to MEF and QNN;100%(30/30)and 96.7%(29/30) isolates were found to be resistant to AMO and CHL,the cross resistance rate was 100% to the two drugs.One of 5 isolates exhibited resistance to PYR.The results suggest that the P.falciparum was susceptible to MEF and QNN,but was resistant to AMO,CHL and S/P. CHL resistant P.falciparum showed a marked cross resistance to AMO,but not to MEF,QNN,and S/P;AMO resistant P.falciparum exhibited cross resistance to CHL,but not to MEF,QNN,and S/P.