The effect of diphenyl vapor on the growth of 10 citrus fruit pathogens was studied by placing crystals of this chemical in the center of potato-dextrose-agar plates in which the fungi were grown. The growth of a few organisms was totally inhibited, some were moderately or only slightly-checked and 1 was apparently stimulated. Likewise, differences in response were observed in spore germination studies. In the presence of diphenyl the spores frequently become enlarged and distorted and often burst. Upon removal of diphenyl from cultures, the organisms resumed growth and spore production; the action of this chemical is evidently fungistatic rather than fungicidal. The control of rot in citrus fruit during transit, storage, and marketing through the use of diphenyl-treated wraps or trays is due to the inhibition of germination of contaminating surface spores, the retardation of growth of exposed hyphae already present on the fruit, and the prevention of new spore formation that often is the source of infections during handling. These expts. show that diphenyl vapors should be effective in checking the development of blue mold rot (Penicillium italicum), Botrytis rot (Botrytis cinerea), Diplodia stem end rot (Diplodia natalensis), green mold rot (Penicillium digitatum), and Phomopsis stem end rot (Phomopsis citri). Diphenyl is not likely to control satisfactorily Alternaria rot (Alternaria citri), anthracnose (Colletotrichum gloeosporioides), brown rot (Phytophthora citrophthora), cottony rot (Sclerotinia sclerotiorum) and Trichoderma rot (Trichoderma viride).