A lipid analysis was performed on the avian malaria parasite Plasmodium lophurae, freed from duckling erythrocytes by immune hemolysis, and on the erythrocytes and plasmas of normal and P. lophurae-infected ducklings. Major lipids of normal erythrocytes were: phosphatidylcholine (40% of total lipids), phosphatidylethanolamine (20%), cholesterol (20%), sphingomyelin (11%), and glycosphingolipids (5%). Major fatty acids of erythrocyte total phospholipids (74% of total lipids) were 16:0 (22%), 18:2 (n-6) (21%), 18.1 (n-7, n-9) (18%), 18:0 (9%), 20:4 (n-6) (9%), 22:6 (n-3) (5%). Erythrocyte phosphatidylcholine was greater than 90% the diacyl form, while phosphatidylethanolamine was approximately 44% alkoxy forms and phosphatidylinositol approximately 11% alkoxy forms. Major fatty aldehydes of phosphatidylethanolamine were 16:0 (47%), 18:1 (23%), 18:0 (14%), and 14:0 (12%). The lipid composition of P. lophurae (plus the parasitophorous vacuole membrane) was qualitatively and quantitatively different from that of the duckling erythrocyte in a number of respects. Major lipids were phosphatidylcholine (40%), phosphatidylethanolamine (36%), cholesterol (8%), phosphatidylinostol (4%), 1,2-diacylglycerols (3%), sphingomyelin (2%), and glycosphingolipids (2%). Diphosphatidylglycerol (approximately 1%) was also detected. The major fatty acids of parasite total phospholipids (86% of total lipids) were more saturated than those of the erythrocyte, and octadecenoic acids were notably elevated: 18:1 (33%), 16:0 (26%), 18:0 (16%), 18:2 (12%), 20:4 (3%), and 22:6 (3%). Parasite phosphatidylcholine and phosphatidylethanolamine were greater than 93% the diacyl form and phosphatidylinositol was approximately 25% alkoxy forms. Major fatty aldehydes of the phosphatidylethanolamine were 14:0 (62%), unidentified long chain forms (24%), 16:0 (7%), 18:0 (4%), 18:1 (3%). The lipid composition of the infected erythrocyte reflected the separate contributions of the erythrocyte and parasite. The major lipids of normal duckling plasma were phosphatidylcholine (33%), triacylglycerols (22%), cholesterol esters (20%), cholesterol (12%), phosphatidylethanolamine (5%), and sphingomyelin (2%). The fatty acids of plasma total lipids were 18:1 (26%), 16:0 (26%), 18:2 (12%), 20:4 (12%), 18:0 (9%), 22:6 (3%). Plasma phosphoglycerides were remarkably lower in C18 unsaturated fatty acids and higher in 20:4 than the erythrocyte phosphoglycerides. Infection of ducklings with P. lophurae caused increases in plasma unesterified fatty acids, triacylglycerols and cholesterol esters, and a notable rise in the 18:1 content of all fatty acid-containing plasma neutral lipids. These findings are compared with those reported for other species of Plasmodium infecting other avian and mammalian hosts.