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The infectious reservoir of Plasmodium vivax and P. falciparum in a malaria endemic region in Sri Lanka was defined in a population of 3, 625 by directly feeding mosquitoes on a sample of infected individuals during a period of 17 months. The malaria case incidence in this population was concurrently monitored. P. vivax gametocyte densities were highest in the youngest age groups, and decreased steadily with increasing age. However, the infectivity per gametocyte appeared to be lower in the younger age groups than in the older ones. There was no significant correlation between the age of patients and their gametocyte densities for P. falciparum, to which this population was only recently exposed, nor was there a discernible trend in the infectivity per gametocyte in different age groups. The average infectivity of patients was lowest in the youngest (0–5 years) and the oldest (> 50) age groups. The contribution made by P. vivax patients in the different age groups to the reservoir of infection was estimated. Patients in the 6–25 year age groups made the largest contribution to the reservoir, followed by those in the 26–50 year age group. Patients in the youngest and the oldest age groups contributed least to the infectious reservoir. When population sizes in the different age groups were taken into consideration, the age groups between 6 and 50 years contributed almost equally to approximately 87% of the infectious reservoir. The reservoir of P. falciparum malaria was very small, being confined to 9% of the patients, and this appears to be a characteristic of epidemic malaria, as was the case with P. falciparum.