Characterization of Plasmodium falciparum strains obtained from three patients interned in the Clinico-Pharmalogical Testing Centre of the Barros Barreto Hospital, Belém, Pará, Brazil, was carried out using cellulose acetate electrophoresis for the two enzymes glucose phosphate isomerase EC.5.3.1.9 (GPI) and adenosine diaminase EC.3.5.4.4 (ADA) and sensibility tests for the two antimalarial drugs, chloroquin and mefloquine. This characterization showed an enzyme variant in one of the patients, and a modification in the chloroquine sensibility level in another; not only in the strains from the original infections, but also in those from each of the relapses.