Blood-samples from children with the sickle-cell trait (A/S heterozygotes) having acute malaria (Plasmodium falciparum) were incubated in vitro under anaerobic conditions, such that the number of cells sickled is a linear function of time. The rate of sickling of parasitised cells was found to be 2 to 8 times greater than that of non-parasitised cells within the same blood-sample, indicating that parasitisation of an A/S erythrocyte by P. falciparum increases substantially its probability to sickle. It is suggested that parasitised cells, once sickled, will be removed more effectively from the circulation by phagocytosis, and that this is the main mechanism whereby A/S heterozygotes are at a selective advantage against subtertian malaria.