The growth of aquatic fungi and fungus-like organisms on the predatory heteropter Notonecta glauca was investigated in laboratory conditions. Water samples were collected from four sites varying in trophicity: Cypisek Spring, Biała River, Supraśl River, and Komosa Lake situated near Białystok. Certain physico-chemical parameters were determined in the samples. The highest contents of carbon dioxide, ammonium nitrogen, phosphates, and chlorides were noted in Biała River. Twenty-eight fungi and fungus-like organisms were isolated from Notonecta glauca, including 6 species of Chytridiales, 2 species of Blastocladiales, and 20 species of oomycetes. Fourteen fungi and fungus-like organisms were identified in Cypisek Spring, 8 in Biała River, 17 in Supraśl River, and 10 in Komosa Lake. Worthy of note is the finding of some aquatic fungi which have been commonly encountered on chitin-containing substrata, namely Aphanomyces astaci, A. amfigynus, Chytriomyces hyalinus, Phlyctorhiza endogena, Podochytrium chitinophilum, Rhizidium chitinophilum, and Rh. nowakowskii.