The first isolation of Penicillium marneffei, obtained from the liver of a rat captured in Viet Nam, was described in 1956. There have been no additional reports of the isolation of this fungus. Recently our laboratory identified this organism in an isolate from the spleen of a patient with Hodgkin's disease. The distinctive features of P. marneffei include the production of the dark red pigment which diffuses into the agar and the ability of this pathogen to exist in dimorphic forms. At 25 C. it develops the structures typical of the genus Penicillium, while at 37 C. the tan, rough colony has a yeast-like consistency, with small, oval or round yeast cells averaging 3 p in diameter mixed with short, hyphal elements. Histologically, the organism may resemble Histoplasm capsulatum.