The inhibition of 23 strains of actinomycetes by 7 strains of fluorescent pseudomonads and vice versa, on 4 different media, was examined. The antagonism of pseudomonads was more frequent and more intense than that of actinomycetes. Inhibition was observed in pseudomonads and actinomycetes of both kinds of strains, antagonistic and non-antagonistic to Fusarium oxysporum. Some media were more favorable to the an-tagonism of pseudomonads and others to that of actinomycetes, hence in identical environmental conditions mutual antagonism between the two counter-partners occurred only rarely.