On surroundings of a branch of the C.N.E.P.A. situated at Km 47 of the Rio-S. Paulo road (Rio de Janeiro) it was reported a rust attacking young plants of Eucalyptus citriodora Hk. in seed-plot and in transplanting boxes. Teliospores not observed in that occasion, showed up later and were found to belong to Puccinia psidii Winter. This species was hitherto unknown to this host. The Uredo sp. found by Gonçalves in the State of Espírito Santo, in 1932, on Eucalyptus sp. was considered the first known Uredinales attacking the plant afore mentioned. Later, the same fungus was found on Eucalyptus in two districts in the state Pernambuco. After examining several herbarium material and consulted with the diagnoses of thirteen Puccinia species recorded upon Myrtacex, twelve from them were found to be morphologicaly identical to the rust on Eucalyptus. Specimen of Eucalyptus rust was sent to prof. G. C. Cummins, Purdue University, and he identified it as P. psidii Wint. His classification was confirmed also by Dr. F. Kern, of Pennsylvania State College, U.S.A. P. psidii is commonly found in several regions of Brazil upon Myrtacex. As a parasite of Eucalyptus it appears on leaves, petioles, and new stem tissues; the rust injuries seedlings making them improper for planting. The rust was not found on other species of cultivated eucalypti in the same site and climate conditions in which E. citriodora was found.