It has been established that host lipids play a unique role for long term survival and life cycle completion in endogenous parasites. Study in our laboratory revealed that the content of total lipids increased in the parasite, Perkinsus marinus, parallel with the increase of cell numbers. Depletion of total lipid, different class lipids and arachidonic acid in media cultured with the parasite was noted. To study lipid utilization by this parasite, we followed the uptake of fluorescent fatty acid phospholipid analogs in the merozoite/meront stage of P. marinus. After 24 hr incubation at 28°C with fluorescent phosphocholine (PC) analog, yellow-gold fluorescence was present primarily in the discrete lipid droplets throughout the parasite. Lipid class analysis using thin layer chromatography (TLC) demonstrated that fluorescence was restricted in the polar (phospholipid) lipid fraction. Further analysis of the phospholipid class composition, fluorescence was noted in phosphatidylserine (PS), phosphatidylinositol (PI), and phosphatidylethanolamine (PE), in addition to PC. This suggests that transformation of PC into PE and PI/PS occurred. Currently we are determining the uptake and metabolism of fluorescent fatty acids and other phospholipids by the parasite.