The influence of an isolate of Epicoccum nigrum and one of its antibiotics, flavipin, on the spores, mycelium, and germ tubes of Monilinia laxa in culture was studied using light and electron microscopy. Epicoccum nigrum and flavipin induced the development of stromata in cultures. Abundant clusters of microconidia of M. laxa were produced on the induced stromata exposed to E. nigrum and flavipin. Deformation of hyphae and germ tubes such as swellings, coilings, and abnormal ramifications were also noticeable under light and scanning electron microscopy after treatment with E. nigrum or flavipin. Transmission electron microscopy revealed cytoplasmic coagulation of the cells and abundant vacuoles and lipid bodies associated with membranes. In some cases alteration and disorganization of membranes was also apparent. Key words: antagonism, antibiosis, flavipin, biological control.