Different densities of saprophytic colonization of rye leaves were obtained in field experiments by spraying with benomyl or water. In 1972 and in 1973, inoculation with Coehliobolus sativus, just after flowering, resulted in 60 % less necrosis on water-sprayed leaves than on benomyl-sprayed leaves. At that time, the natural mycoflora of the water-sprayed leaves amounted to 10 000 and 3000 propagules per cm 2 leaf surface in 1972 and 1973, respectively. The benomyl treatment reduced the colonization to 1200 and to 400 propagules per cm 2 in 1972 and 1973, respectively, which implied an apparent reduction of the antagonistic capacity of the mycoflora. In 1974, the saprophytic colonization of the water-sprayed leaves reached only 500 propagules per cm 2 just after flowering, a population density not high enough to be antagonistic. Benomyl had a differential effect on the phyllosphere fungi: Cryptoeoceus spp. were not affected, Sporobolomyces roseus and Cladosporium spp. were reduced to less than 6~ of the control populations, and Aureobasidium pullulans was eliminated. When later in the seasons of 1972 and 1974 the 'white yeasts' on the benomyl-sprayed leaves reached population densities of 6500 propagules per cm 2 and more, inoculation of these leaves resulted in a necrosis similar to that of the water-sprayed leaves with higher population densities. Above a population density of 6500 propagules per cm z no correlation existed between the density of the antagonists and their action on C. sativus.