During the studies on malaria chemoresistance, we noted great variations in parasite density of Plasmodium falciparum between screening in the morning and final selection in the afternoon, in asymptomatic people. To better understand this phenomenon, we conducted a study in october 1987 on primary school children in a village near the city of Bobo-Dioulasso, at the peak malaria prevalence. We performed 3 blood-smears at 8 a.m., 2 p.m. and 8 p.m., on Day 0 and Day 4, to an initial number of 86 children, aged from 6 to 9 years. By the end of the study 44 children remained who fulfilled the inclusion criteria Among them 35 showed a parasitaemia on Day 0 and 9 remained negative. On Day 4, 28 were positive and 16 remained negative. Of the 35 children positive at entry to the study 16 remained continuously positive, the others were negative on at least one occasion. Of the 28 children positive on Day 4, 14 remained continuously positive. For the 16 people with a parasitaemia continuously positive on Day 0.7 (43.7 p. cent) became spontaneously negative on Day 4. But considering the small size of our sample, the analysis of the nycthemeral variation and of the variation between the two days did not show a significant difference. Further studies involving a greater number of blood-smears during a longer period and concerning more people, should be conducted. The possibility of spontaneous negativation of the parasitaemia without drug absorption shows that there are some cases of false malaria chemosensitivity that are declared when the in vivo tests are not coupled with in vitro tests.