The response of Plasmodium falciparum to isoleucine withdrawal is dependent on the stage of progression through the intraerythrocytic cell cycle. The human malaria parasite Plasmodium falciparum appeared to stall its growth upon removal of the amino acid isoleucine from cell culture medium. We have re-examined this phenomenon and found that isoleucine-deprived parasites never truly exit the cell cycle but instead continuously grow at a markedly reduced pace. This slow growth occurs only if isoleucine is removed prior to the onset of DNA replication. Once S-phase initiates, the parasite is insensitive to isoleucine removal and progresses through the cell cycle normally. This response is reminiscent of the nutrient-dependent G1 cell cycle checkpoints described in other organisms.