The antifungal and antibacterial properties of a series of 70 diarylamidine derivatives were evaluated. Several of these compounds exhibited considerable antimicrobial potency. A survey of the structure-activity relationship demonstrated that minor structural variations resulted in significant changes of antimicrobial activity. In general, the structural features required for antifungal activity coincided with those required for antibacterial activity. Both the antifungal and the antibacterial properties of the diarylamidines depended on the presence and the positions, of both amidino groups, on the nature of the central bridge connecting the two aryl moieties, and on the nature of these aryl residues (preferably indole). The most active compound was evaluated for its activity against Candida albicans infection in mice.