During the development of the uredial and telial stages of the rust, Puccinia poarum, on leaves of Poa pratensis, lipid accumulates in and around the infected tissues to a level more than double that of uninfected leaves. Microscopically, the infection on Poa differs from that on the alternate host, Tussilago furfara, in the less extensive development of fungal tissue, the much more frequent occurrence of haustoria and the greater amount and persistence of lipid globules in host cells. Urediosori contain significantly more triglyceride and sterols than healthy tissue and teliosori contain more neutral lipid of all classes. As on Tussilago. there was a rapid disappearance of chloropiast glycolipids and phosphatidyl glycerol in the early phases of growth on Poa, accompanied by increases in phosphatidyl choline, phosphatidyl ethanolamine and phosphatidyl serine, which are attributed primarily to synthesis of fungal membrane lipids.