Two three weeks field studies were performed in Morondava (West Coast) and Nosy Be (North West Coast) respectively in January and April 1983. Among 110 malarial patients recruited through passive detection, 70 in vivo tests and 37 in vitro tests were performed. In vivo, within 7 days 10 mg/kg chloroquine failed in 8 of the 21 treated patients, 25 mg/kg chloroquine failed in 1/43 and none of these patients treated with 10 mg/kg amodiaquine failed. In vitro, 5 among 37 isolates had chloroquine IC50 more than 250 nM/1 which is predictive for resistance with the semi-microtest. All isolates were sensitive in vitro to low doses of amodiaquine, piperaquine and dichlorquinazine.