Oidium sp. has been recovered as an endophyte in Terminalia catappa (tropical chestnut) in Costa Rica. The volatile organic compounds (VOCs) of this organism uniquely and primarily consist of esters of propanoic acid, 2-methyl-, butanoic acid, 2-methyl-, and butanoic acid, 3-methyl-. The VOCs of Oidium sp. are slightly inhibitory to many plant pathogenic fungi. Previous work on the VOCs of Muscodor albus demonstrated that besides esters of small organic acids, a small organic acid and a naphthalene derivative were needed to obtain maximum antibiotic activity. Thus, the addition of exogenous volatile compounds such as isobutyric acid and naphthalene, 1,1′-oxybis caused a dramatic synergistic increase in the antibiotic activity of the VOCs of Oidium sp. against Pythium ultimum. In fact, at elevated concentrations, there was not only 100% inhibition of P. ultimum but killing as well. In addition, a coculture of Muscodor vitigenus (making only naphthalene) and Oidium sp. plus isobutyric acid produced an additive antibiosis effect against P. ultimum. The biological implications of multiple volatile compounds acting to bring about antibiosis in nature are discussed.