Eleven species of wintering waterfowl were trapped on the Welder Wildlife Foundation, San Patricio County, Texas, between October, 1976 and May, 1977. Blood films were made from 580 ducks. Leucocytozoon simondi, Haemoproteus nettionis, Plasmodium circumflexum, and a microfilaria were found in three species. These blood parasites occurred in 70 lesser scaups (Aythya affinis), 12 blue-winged teals (Anas discors), and 3 ring-necked ducks (Aythya collaris). There was no difference in the rates of infection between the sexes of the three host species. Adult blue-winged teal and lesser scaup were more heavily infected than juveniles. Juvenile ring-necked ducks had more parasites than adults. Blood parasites were found in 85 of 580 ducks (14.6%) throughout the wintering period.