In October and November 1988 we carried out in vivo Plasmodium falciparum sensitivity tests for chloroquine in the town of Dakar, where malaria is endemic with increased transmission during the rainy season (July to October). 25 mg/kg/p.c of chloroquine has been administered to 35 patients on three consecutive days, according to the WHO protocol. Clinical and parasitological controls were carried out on D1, D2, D3, D4, D7 and D14. We observed two therapeutic failures: one on D7 among the 32 patients examined during that period, and one on D14 among the 24 patients who were present at control. The important drug pressure and population movements in Dakar are sources of chloroquine resistance which should receive special attention in terms of control and surveillance of this emerging problem.