The importance of the Hyphomycetes Fungi Imperfecti stems from their crucial role in purifying both flowing and standing waters, which can balance the effects of eutrophication. The purpose of the present work was to analyze Hyphomycetes populations in the 11 fish ponds, each with different leaves and dead plant composition. The research was carried out in autumn 2001 and spring 2002. We investigated water samples from fish farms in Northeastern Poland. In breeding the fungi some water plants and bulrushes from the ponds were used. We discovered 79 species of Fungi Imperfecti. Eight of which appeared to be new to Northeastern Poland. In addition, pathogenic species: Alternatria sp. and Fusarium sp. were also found. The most optimum conditions for the Hyphomycetes representatives were found in the ponds overgrown by numerous water plants and bulrushes. It confirms that saprophytic Fungi Imperfecti play an active role in decomposition of dead plants, and purification of each pond’s water.