Drug resistance tests were performed with 171 strains of P. falciparum obtained from the Amazon Region, using chloroquine, mefloquine, amodiaquine and quinine. Tests lasted about 24 hours and the drug solutions were prepared just before use. Results show high percentage of resistance to chloroquine (83%) whereas nearly all strains were shown to be sensitive to mefloquine (97.7%). For both amodiaquine and quinine sensitivities were of 51.0% and 56.5% respectively. The present study demonstrates that resistance to mefloquine may be starting in Tucuruí, Pará.