Blood smears from 837 mallard ducks (Anas platyrhynchos), collected during January and February from various locales in Washington during the period 1974-1977, were examined for hematozoa. Of these, 339 (40.5%) harbored blood parasites: Leucocytozoon simondi was the most common, occurring in 24.4% of the birds, followed by Haemoproteus nettionis (23.4%). Microfilariae were found in 14 birds (1.7%).