A study is reported of the primary parasitemia in ducklings naturally infected with L. simondi as a consequence of farm exposure in northern Michigan. Immature gametocytes were found to attain a maximal concentration at about the fourth day of patency. Mature parasites were most commonly observed for the first time on the fifth day of patency, their number increasing with the simultaneous decline in immature gametocytemia. Mature gametocytes generally reached their peak density by the ninth day of patency, while the absolute peak of parasitemia (total of all forms) occurred on the sixth day of the patent period. The primary parasitemia usually terminated within 30 days of the first appearance of parasites. A number of variations in the relative proportions of immature to mature parasites have been noted ,and are briefly discussed. There is suggestive, but as yet inconclusive, evidence that the immunity to this infection may be a very rapidly developing one.