In vitro, antibiotic effects of the culture filtrates of 29 species of aquatic hyphomycetes against Gram-negative and Gram-positive bacteria, yeasts and hyphomycetes were studied. The culture filtrates of 38% of isolates were active in at least one of our tests for biological activity. Fifteen species inhibited growth of bacteria and four demonstrated antifungal activity. Filtrates of 35% of the isolates were antibacterial. About 50% of the isolates, those with the largest in vitro inhibitory activities (Dimorphosphora foliicola, Flagellospora sp. and Mycocentrospora sp.) were obtained from wood. Filtrates of Articulospora tetracladia were not only active against Gram −ve bacteria, but were also antiviral against bacteriophage T4 and influenza virus.