Stationary liquid cultures of a fungus isolated in this laboratory as an air contaminant excreted into the medium a purple pigment which could be extracted with ether and purified by crystallization from aqueous methanol. Analyses and molecular weight estimations on the pigment and derivatives best fitted a molecular formula C14H10O8. A tetraacetate and tetramethyl ether were prepared, and reductive acetylation gave a colorless dihydrooctaacetate with an ultraviolet absorption spectrum similar to that of dibenzoquinone leucoacetate. These properties suggested that the pigment might be oosporein, a substance of known chemical structure produced by Oospora colorans and by Chaetomium aureum. Direct comparisons with authentic Bamples confirmed the identification. Infrared, ultraviolet, and visible absorption maxima of oosporein and of the derivatives mentioned above are reported. The fungus (PRL 1699) belongs to the family Moniliaceae. As far as can be determined from available literature, it is a member of the genus Acremonium and a complete study of its morphology and development will appear as part of a comparative study of the various species of Acremonium, a number of which are known to produce purple pigments diffusible in the agar medium on which they are growing. Although oosporein has been reisolated recently from Chaetomium and other species, to the authors'' knowledge, its production by an Acremonium has not previously been described.