The authors analyzed the occurrence of zoosporic fungi and straminipilous organisms in relation to environmental factors in the main bed of the Narew River and its three tributaries (Awissa, Turośnianka, and Kurówka) in two vegetative seasons: spring and autumn. The bait method used to isolate microorganisms helped to distinguish various physiological groups, such as phytosaprotrophs, zoosaprotrophs, and parasites of plants, animals, and fungi. Forty taxons that were determined were more frequently found in the main current of the Narew River than in its tributaries. The major factors limiting the species composition of zoosporic fungi and straminipilous organisms in the sampling locations included hydrochemical parameters, such as CO₂, nitrites, nitrates, ammonium nitrogen, and phosphates that showed negative correlation, pH, and O₂ being positively correlated with the number of the microorganisms studied.