A study of Tabebuia ochracea ssp. neochrysantha, a plant traditionally used in the Amazon against malaria, was pursued. Bioactivity was tested in vitro against Plasmodium berghei and Plasmodium falciparum (FcB2 chloroquine-resistant strain). Inhibitory activity was determined by measuring parasite 3H-hypoxanthine incorporation. Fractionation of the chloroformic extract of P. ochracea (inner stem bark) afforded five furanonaphthoquinones. The highest antimalarial activity against P. berghei was given by a mixture of two compounds which could not be separated, but the isomeric structures of 5- and 8-hydroxy-2-(1'-hydroxy)-ethylnaphtho-[2,3-b]-furan-4,9-dione (1 and 2) were determined from spectroscopic data. The 50% inhibitory concentration (IC50 ) values obtained with the mixture of compounds 1 and 2 were 1.67 3 10–7 M for P. berghei and 6.77 3 10–7 for the FcB2 chloroquineresistant strain of P. falciparum. For the former parasite, the IC50 value for chloroquine was 5 3 10–8 M. That for P. falciparum was 1.1 3 10–7 M. These results indicate that the furanonaphthoquinones isolated from T. ochracea are potential antimalarial compounds.