The in vitro activity of artemether against 63 African isolates of Plasmodium falciparum from Libreville, Gabon was evaluated using an isotopic drug susceptibility semi-microtest. The 50% inhibitory concentration (IC50) values for artemether were in a narrow range from 0·8 to 34·8 nm (mean IC50 5·0 nm) and the 95% confidence interval (CI95%) was 3·6–6·3 nm. In vitro decreased susceptibility or resistance were observed with artemether (14%), to chloroquine (90%), to quinine (32%). Isolate susceptibility to amodiaquine and halofantrine was high i.e. 100% and 98%, respectively. There was a significant positive correlation between responses to artemether and amodiaquine (r2=0·45, P<0·001), artemether and chloroquine (r2=0·36, P<0·001), artemether and quinine (r2=0·31, P<0·001), and artemether and halofantrine r2=0·19, P<0·01). Positive correlation between these drugs suggests in vitro cross-resistance or at least common features in drug uptake and/or mode of action or resistance.