Macrocysts of a protozoan parasite, Sarcocystis sp., were found in 28 of 173 American black ducks (Anas rubripes) examined during winters 1984–1985, 1985–1986 and 1986–1987 in south coastal New Jersey (USA). No macrocysts were detected in 80 juvenile black ducks. In adults, the prevalence of macrocysts increased from 3% (1 of 37) in 1984–1985, to 36% (12 of 33) in 1985–1986, and 65% (15 of 23) in 1986–1987. This increase could result in a greater number of harvested birds being discarded, or a change in the attitudes of waterfowl hunters towards black ducks.