The authors investigated the influence of the green hydra (Chlorohydra viridissima) on occurrence on aquatic fungus species in the water of three water bodies of different trophy. The seeds and snake exuviae were used as baits. Radioactive carbon (¹⁴C) was used for the measurement of photosynthesis processes, extracellular products by symbiotic algae of green hydra and assimilation of those products by fungi. A total of 76 fungal species were found growing on the baits, including 29 belonging to the kingdom Fungi, 46 to Straminipila and one to Plasmodiophoromycetes. More fungi were found to grow in the containers with green hydra (Hy) than in the control containers (Co) in water from all water bodies. The mean ratio of Hy/Co ranged from 1.80 (Lake Wigry) to 2.20 (River Czarna Hańcza). The effect of green hydra on the abundance of aquatic fungus species is caused by dissolved organic matter (extracellular products) produced during photosynthesis by Zoochlorella sp., a symbiont of the green hydra, and excreted into water environment (mean 28.8% of carbon fixation). Mean ¹⁴C fixation in symbiotic algae of green hydra was 4.28 mg C g^-1 dry weight hydra h^-1. Those excreted organic substances serve as nutrients for aquatic fungi. The mean value of extracellular products assimilated by fungi was 2.12 mg C g^-1 dry weight mycelium h^-1.