The chronobiology of the gametocytes of P. yoelii was studied in Percoll-glucose synchronized infection in the mouse. The gametocyte developmental cycle consisted of 4 successive stages: stage 0 maturation took 27 hours from merozoite invasion, stage 0 to stage 1 lasted 6 hours, stage I to stage II and stage II to stage III lasted 3 hours each. Stage 0 gametocytes were found to sequester in small peripheral capillaries, and the number of oocysts in mosquitoes was related to the number of stage 0 gametocytes ingested.