To determine the role of fungi in decomposition, sediment from a eutrophic lake and a bog was treated with antibacterial antibiotics or with antifungal antibiotics. Sediment treated with antibacterial antibiotics showed a decrease in biochemical oxygen demand and a decrease in direct and pour-plate bacterial counts when compared with controls. Sediment treated with antifungal antibiotics showed an increase in biochemical oxygen demand and an increase in pour-plate and direct bacterial counts when compared with controls. Two species of lake fungi inhibited the growth of a mixed culture of lake bacteria on agar plates. The results infer that fungi inhibit the growth of bacteria in aerobic lake sediment.