Antifungal phenols, absent from fecal pellets of germ-free desert locusts, Schistocerca gregaria, were found in the fecal pellets of locusts which had been monoassociated with Pantoea agglomerans. In addition, gut fluid from monoassociated insects was inhibitory to germination of conidia of Metarhizium anisopliae, whereas gut fluid from germ-free locusts was not. It is concluded that P. agglomerans probably produced the phenols. The presence in fecal pellets from monoassociated insects of only one of the three phenols identified from conventional insects and the occurrence of another fungitoxic: phenol suggest that in conventional insects a number of bacteria including P. agglomerans cooperate in the production of the phenols. The antifungal phenols are also selectively bacteriocidal and it is suggested that the toxins may have a wide role in host defense against pathogenic microorganisms and a significant impact on the composition of the gut flora. (C) 1995 Academic Press, Inc.