Abstract: Streptomyces sp. are of the most attractive sources of biologically active compounds. In the recent decades, they have attracted high interests as biocontrol agents. In search of finding such principles, in vitro suppression of Alternaria solani was studied by use of Streptomyces antagonists. A. solani was sensitive to several species mainly Streptomyces plicatus, S. olivaceus and two unidentified actinomycetes. Activity versus time was monitored in S. olivaceus in solid and shaked cultures. Ongoing goals of this research include isolation, characterization and identification of the active metabolites. Future goals include identification of active genes for use in development of recombinant DNAs in transgenic hosts bearing elevated resistance to infections by Alternaria sp.