The occurrence and growth of aquatic zoosporic microorganisms on the tick Dermacentor reticulatus F. were investigated in laboratory conditions. Water samples were collected from sites of the Narewka River and natural ponds in Białowieża National Park. Some physico-chemical parameters were analyzed as well. The highest contents of ammonium nitrogen and phosphates were found in the water of the Narewka River. Forty-six species of zoosporic microorganisms were isolated from Dermacentor reticulatus F. ticks, including 8 of the Chytridiomycetes class, 1 of Plasmodiophoromycetes, 35 of Peronosporomycetes and 2 Zygomycetes. The highest number of species was identified in ponds situated in the village of Białowieża (30), the lowest (22) in the Narewka River outside the village. The most common aquatic Peronosporomycetes included Achlya americana, Ac. prolifera, Aphanomyces laevis, Ap. irregularis, Dictyuchus monosporus and Saprolegnia ferax, found on the fragments of most ticks in the river and pond water. Cluster analysis of the investigated parameters revealed that the environmental factors determining the number of aquatic zoosporic microorganisms was oxidability in the Narewka River, and nitrate and nitrite in the ponds of Białowieża village.