To further document the phenomenon of Plasmodium falciparum resistance to mefloquine formerly described in Cameroon, complementary in vivo and in vitro studies were conducted. Two hundred six P. falciparum isolates were studied in vitro using an isotopic microassay with mefloquine solutions prepared daily. Using the cutoff limit of 30 nmol, 26 (20%) of 133 isolates from the northern part of the country were defined as being resistant to mefloquine. In contrast, only one of 73 isolates collected in the southern part of the country was resistant. In vivo 7-day assays were performed in the northern area in 57 asymptomatic P. falciparum carriers (age range 1–10 years) who were given a single 25 mg/kg dose of mefloquine (Lariam). Among 46 cases in which followup was possible, P. falciparum asexual parasites were cleared within five days in 38 cases, by days 6 and 7 in two cases, and remained detectable up to day 7 in six cases. Thus, these latter patients have a RII–RIII level of resistance by in vivo criteria. No resistance was found in 40 additional patients studied similarly in the southern region. These observations were made before any mefloquine drug pressure was exerted in the country, but results of cross-resistance and drug consumption studies support the hypothesis that in the northern region, where a close correlation (r = 0.67) was found between the response to quinine and mefloquine, the more frequent use of quinine may have induced a primary quinine resistance and a secondary mefloquine resistance (without chloroquine resistance). Chloroquine drug pressure in the southern region, which is 2.5 times higher than in the northern region, possibly led to a primary chloroquine resistance and a secondary quinine resistance (without mefloquine resistance).