Although trophozoites of Giardia lamblia have not been demonstrated to possess the capacity for synthesis of phospholipids, these protozoan parasites would be exposed to fatty acids within the human small intestine. We have evaluated the metabolic incorporation of arachidonic and palmitic acids by Giardia trophozoites. Trophozoites (2.25 X 10(6)) were incubated with 12 nM [3H]fatty acid for up to 60 min. Uptake of [3H]arachidonate by trophozoites was rapid, increasing from 37% at 1 min to 65% at 10 min. Uptake of palmitate was rapid but less extensive. In contrast to palmitate, almost all of the trophozoite-associated [3H]arachidonate was esterified into phospholipids and neutral lipids. By 1 and 60 min 37% and 82% of [3H]arachidonate, respectively, were incorporated into phospholipids, including phosphatidylinositol and phosphatidylcholine. Peak incorporation of [3H]arachidonate into phosphatidylcholine (30 mmol [3H]fatty acid (mol phospholipid)-1) occurred at 60 min; whereas incorporation into the pool of phosphatidylinositol, which accounted for only 4% of trophozoite phospholipid, was maximal at 10 min (190 mmol [3H]fatty acid (mol phospholipid)-1) and declined significantly thereafter as arachidonic acid was released from phosphatidylinositol. Therefore, Giardia trophozoites not only utilize exogenous fatty acids in the formation of glycerolipids but also preferentially incorporate arachidonic acid into a metabolically active pool of phosphatidylinositol.