The fecal excretion of phospholipids was determined in three adult males during the last 4 days of eight dietary periods of 8–16 days on high corn oil and butterfat diets (35–60% of calories from fat). The phospholipids were isolated, identified, and quantitatively estimated by a combination of column, thin-layer, and gas chromatographic techniques. On both high fat diets the chief components of the fecal phospholipid mixtures were tentatively identified as phosphatidyl glycerol, phosphatidyl ethanolamine, phosphatidyl choline, and phosphatidyl inositol. The output of total phospholipids in three subjects on butterfat ranged between 33 and 168 mg/day. Substitution of hydrogenated corn oil (45% of calories) and corn oil (60% of calories) for butterfat (35–60% of calories) led to increased excretion of phospholipids, 300 and 430 mg/day, respectively, for hydrogenated and refined corn oil. Addition of sitosterol to a butterfat diet also led to increased phospholipid output (208 mg/day), as did the mixing of butterfat and corn oil (180 mg/day). The changes in the total output of the phospholipids were accompanied by alterations in the proportions of the individual phospholipids as well as of the component fatty acids. It is concluded that the ingestion of corn oil or plant sterol leads to increased fecal output of phospholipids, when compared with butterfat and fat-free diets. A change in the activity or the population of the intestinal flora is suggested to explain this observation.