In vivo testing of Plasmodium falciparum sensitivity to chloroquine was carried out in four rural sites of differing socio-geographical environment in Côte d'Ivoire. Of a total of 1282 patients of all ages with fever or previous history of fever, 649 were slide positive, with 435 patients with a pure P. falciparum infection; 191 fulfilled all the criteria for inclusion in this study, and 113 completed it. Treatment failure rates ranged from 9.7% (Djébonoua) to 38.1% (Tiéviéssou), and were most often associated with higher degrees of resistance (RII = 54.2%; RIII = 37.5%). Blood chloroquine levels measured by ELISA test suggest that many people take chloroquine routinely; furthermore 37.5% of resistance cases occurred in subjects who had high blood chloroquine concentrations on day 0. Twenty-three out of 24 cases of resistance were found in children under 7 years of age. Nearly all children with persisting parasitaemia were afebrile on day 7, even those (7/8) with RIII resistance. Children aged < 7 years represent the the best sentinel group for monitoring P. falciparum sensitivity to chloroquine in Côte d'Ivoire.