During a 3-year study of the epizootiology and the phenomenon of relapse in Leucocytozoon simondi infections of ducks (Am. J. Hyg. 56 :39-57, 101-118, 1952), groups of hatchery-bred White Pekin ducklings were exposed to natural infection on farms in the vicinity of the University of Michigan Biological Station, Cheboygan, Michigan. Birds that contracted and survived L. simondi infections were shipped to Baltimore at the conclusion of each summer (1948-49-50) and their infections were followed by means of thin films taken 3 times weekly during the ensuing winter and spring months; in 1948-49, each film was examined microscopically under high-dry magnification for 15 minutes, while during the following 2 years blood films were each examined for 5 minutes. Each winter, usually during the months of December-February, microfilariae (species not determined) appeared in the peripheral blood of some of the birds that had acquired L simondi infections during the preceding summer. By the time the project was concluded in the spring of 1951, the onset of microfilaremia had been observed in a total of 16 such birds. Since the literature is almost devoid of information pertaining to the duration of the prepatent period in filarial infections, the pertinent data for the 16 cases are presented in this note. Although the 16 ducklings are treated here as a single group, 5 came from among those exposed during the summer of 1948, 6 from the series of 1949, and 5 from the 1950 series, All had been hatched in May of the year they came under study and all were purchased from the same commercial source at Alma, Michigan. They were shipped from the hatchery to Chehoygan by rail and were then protected from insect bites prior to farm exposure and again after removal from the farms. The periods of exposure ranged from 8 to 12 days (during July) in the case of 3 ducklings, and 2 months (July and August) in the remaining 13 birds. The 16 ducks became microfilaria-positive as follows: 4 in December, 5 in January, 6 in February, and 1 in March. Dating from the beginning of the exposure periods to the first appearance of microfilariae, the prepatent period ranged in length from 6 to 9 months with a mean of 7.2 months. Six of the ducks were examined at necropsy for adult filariids but none were found despite painstaking effort. It does not seem at all likely that any of these birds could have been infected while stiIl in the hatchery at Alma, although this possibility cannot be excluded with certainty. However, even allowing for this, the error in estimating the length of the prepatent period cannot be more than 6 weeks since the ducklings were of that age when received from the hatchery. A further source of possible error rests in the fact that the data are based solely on thin-film examinations while more appropriate methods might have disclosed microfilaremia somewhat earlier.