Macrocysts of the protozoan parasite Sarcocystis spp. occurred in 34 of 241 (14%) adult and 6 of 70 (8.6%) immature mottled ducks (Anas fulvigula) that were collected in southwestern Louisiana from 1987 to 1989. The prevalence of Sarcocystis spp. among immature ducks was higher than previously reported, possibly because of a longer exposure period. No differences in carcass composition variables were found between heavily infected and uninfected adult mottled ducks during the winter period of the annual cycle. We concluded that macrocysts of Sarcocystis spp. did not influence carcass composition and probably exert negligible effects on survival of parasitized mottled ducks during winter.